In all honestly, all my dealings with Ness and Lucas have made them play practically the same. Yes some minor things were different, but overall the same strategies worked and the same ways to get back to the stage, KO opponents work.
and Yes, I forgot Luigi. At least his B moves are definitely more different than Lucas and Ness.
Ness and Lucas are not clones, the only thing that is similar is the way their specials are activated; these have different properties that make them unique.
Lets compare the special moves for these two characters, to highlight the differences. (These are from Aevin's great thread)
http://www.smashboards.com/showthread.php?t=149444
Side B
Ness
PK Fire (Forward + B) - (6%, 2%)
A short-ranged projectile which creates a pillar of flame on contact, repeatedly hitting opponents for small damage and producing a stunning effect. In the air, the fire will travel at a 45 degree angle down from Ness. Great for surprise attacks or countering chasing opponents in the air. Also great for ledge guarding, and the stun effect sets up easily for other moves such as grabs, spikes or smash attacks. The initial bolt does 6% damage, the pillar hits each do 2%.
Lucas
PK Fire (Forward + B) - (8%)
Lucas's PK Fire creates a single explosion when it hits, sending the opponent flying away with admirable knock back. Unlike Ness's, it travels straight forward whether in air or on the ground. Great for keeping opponents at a distance, especially when used with B-sticking (covered in advanced strategies).
As you can see here, the first thing that is noticeable is that Ness' PK Fire hits multiple times, and this stuns your opponents, allowing for an approach, or to set up other attacks, something that Lucas can't do as easy, as his PK Fire sends the opponent back in one fire blast. Ness' also travels at a diagonal down angle when in the air, so you can use this to attack opponents directly beneath you, and this allows for Ness to have a safer airgame, and to help fend off up smashers below you, something which Lucas can't do.
Up Specials (cover both versions of the move)
Ness
PK Thunder (Up + B) - (About 8%)
A very quick projectile which can be directed using the controller. On contact, it deals damage and sends the victim upwards. When steering, Ness's thunder can't make very sharp turns, but travels quickly. Unlike Lucas's version, Ness's hits only once before disappearing, unless you "tail whip" opponents by hitting with the trail behind the thunder ball. This stuns them for a second, and can foul up their recovery in certain cases--or aid in yours. Tail-whipping does 1% damage per hit. By steering the thunder into Ness, you can perform the PK Thunder 2.
PK Thunder 2 (Up + B, steered into Ness) - (21-25%)
When PK Thunder is steered into Ness, there's an explosive effect which propels him forward like an electrified human cannonball. This can be used for recovery, or as an attack. This move KOs at low percentages (around 70% in some cases). When used as a recovery move, it has some weaknesses. In Brawl, the tail has been extended, so sometimes when opponents jump at you they'll be stunned, but there's still the danger of them attacking or leaping into the thunder to cancel it, causing Ness to fall to his doom. The initial blast when the thunder first strikes him has armor (invincibility?) frames and KO's as low as 50%, while the later hits of the move KO above about 120%. Hitting projectiles or opponents halves the distance Ness travels, so be careful when using this for recovery.
Lucas
PK Thunder (Up + B) - (About 10%/hit)
A thunder projectile which can be steered using the controller. It travels slowly, but can make sharp turns. When it hits opponents, it stuns them and passes through, enabling it to be curved back around for another hit. The tail whip deals 1% damage. Steering this move into Lucas creates PK Thunder 2.
PK Thunder 2 (Up + B, steered into Lucas) - (8-43%)
When the thunder hits Lucas, he is propelled forward, surrounded by electricity. It can be used for recovery, and goes much farther than Ness's version. Lucas's PKT2 hits multiple times. The first and the last hit can KO at high percentages, and if every hit connects the move does about 43% damage. Lucas's recovery is more difficult to steal, because opponents can't intercept it with their bodies. They can still attack to cancel the thunder, however. By the way, I LOVE hitting with this move! BBZZZZZZZZZZAP!
When using PK thunder normally, Ness can't make it pass through his opponents, meaning he can only score one single hit with this move, unlike Lucas, who can make it go through his opponents. Lucas also has an easier to control PK Thunder, so the multi hit ability combined with the easy steering makes Lucas' better when using it normally.
However, when using PK Thunder 2, it is clear that Ness has the best offensive move, as it deals a single blow that can kill at low percentages depending on where you hit, and it deals good damage, whereas Lucas just scores a multi hit that can deal more damage, but has nowhere near as much KO potential. The downfall to Ness is that it is more easier to intercept, and to gimp, whereas Lucas has an easier time using this move to recover, and if you put his tether into consideration, Lucas does have different recovery options to Ness.
Down B
Ness
PSI Magnet (Down + B)
A field surrounds Ness, absorbing most energy-based projectiles. Comes out much quicker than in Melee, and replenishes 1.5 times the damage the attack would have dealt, but has slightly better range than Lucas's version. The absorption maxes out at 30% per hit. Upon deactivation, it creates a wind effect which can repel projectiles or move certain things a bit. See the advanced strategy section for more on this effect. Psi Magnet can also can be used to stall falling.
Lucas
PSI Magnet (Down B) - (9%)
A field which absorbs energy-based projectiles. In spite of its appearance, this move protects Lucas from all angles. If a projectile is fired from the back, Lucas turns to absorb it, and if he's under fire from both directions, he will still absorb even though he's unable to turn around. Lucas's version has some added advantages over Ness's. It activates and deactivates more quickly, heals 2 times the projectile's damage, and can be used as an attack. The attack activates when the barrier is DEACTIVATED. It's very quick, centered around Lucas's front, and can KO at high percentages. The absorption power maxes out at 30% damage recovery per hit.
The most obvious difference here is that Lucas can use his version of this as an offensive move, something which Ness can't do. However, Ness' barrier has more range, and comes out quick.
This list shows how much each character recovers from absorbing using the Magnet=
Neutral B
Ness
PK Flash (Neutral B) - (9-37%)
A green projectile which can be steered left or right. It explodes in a storm of electricity when the B button is released, and the longer it is out, the more powerful it becomes. At full power, it easily KO's opponents. Uncharged, it sends a quick burst above Ness's head which can be used for protection or juggling.
Lucas
PK Freeze (Neutral B) - (10-19%)
A projectile which can be steered left and right using the controller. When the B button is released, the projectile explodes in a small burst of ice that can freeze opponents. More powerful when out for a longer period of time before detonation. This attack is much faster than Ness's PK Flash, and has greater horizontal distance. This move can KO at high percents, (above 150%).
Ness' neutral B is far more powerful than Lucas', but Lucas' is much more faster, and can also freeze his opponents, but it can't KO as easily as Ness can. Ness can also use it to juggle, and to set up other combos.
There is no point going into standard moves, as most of them are completely different.
EDIT= lol, SirKibble summed it up nicely. Didn't see that there XD.
I wouldn't mind 6 players, they might have to make some stages a bit bigger (Smashville) and "with anyone" might be annoying if you want less than 6 player (if anyone goes on that anymore) but I think it would be really fun. 
