I don't know why people are saying Lucas is bad at comboing. I have nothing against Ness, from my experience Lucas seems overall better. The comparison isn't really necessary though, as the only thing that really connects them are their B moves, and the A moveset is very different.
My input though:
PK Fire: Hard to say, Ness' PK Fire is a setup, but IMO the diagonal angle in the air sucks. For Lucas, PK Fire serves as a good mid-range move when shorthopped.
PK Flash/Freeze: TBH I think neither of them will be used a lot in competitive play, even PK Freeze which is faster is still too slow to really hit good opponents.
PSI Magnet: From what I know Lucas takes this hands down. His Magnet does damage, is very fast, absorbs 2 times the damage of the attack and also covers both sides. I don't know if Ness has any special effec,t but it better be good to compete with that =)
PK Thunder: Here's where it gets iffy: Lucas PKT2 covery more distance and is easier to control. Ness' is faster, and his PKT2 is one strong hit as opposed to Lucas' multiple hits. Damagewise a PKT2 seems about the same (I think~27% for Lucas if he hits bulls-eye), but Ness' PKT2 has more knockback. However, despite the huge tailwhip that Ness' PKT has, Lucas does more damage. I had people takign 20-30% of damaging by simply passing through them with PKT, hitting them multiple times. It's hard to say here, but I prefer Lucas' version.
Grabwise, Lucas has a better grab, but Ness has better throws. I know Ness has more KO power with his throws, but Lucas can KO quite well with them at 120+ %, which actually puts him above most of the cast. He can combo off his downthrow at lower percents. Overall I think Ness takes this, but do not underestimate the Rope Snake =P
Smashes are very different actually. The only really comaprable ones are the Stick and the Bat, and hands down I prefer the stick. The bat has more range and more damage, but the stick comes out a lot faster, and on top of that, seems to have more knockback.
The other 2 smashes are extremely different and I haven't got that much experience with Ness, but there's a lot more to Lucas' smashes than it seems: D-Smash hits three times (At lower percents, the first 2 hits can actually connect on the same enemy for ~30%), it has a very deceptive hitbox that hits behind Lucas as well and it's probably the Downsmash that hits below your character the most, so great at ledges. The downside? It covers both sides, but the back only slightly, and it's slow.
The U-Smash shouldn't be spammed. It's slow and laggy, but with huge range and has more power than anything Ness can offer XD The perfect punisher and nice in some mindgames, but again, really hard to compare to the Yoyo. the good thing about both Lucas' D-Smash and U-Smash is that they can't really be spotdodged/airdodged since they stay out for too long.
The aerials are very different too. Lucas' fair has lower range, but is lightning fast and good for combos at lower percents, the Bair has huge range, a sexy spike and the weak hitbox can combo even at higher percents. Up-Air looks a lot like Ness' though, good damage (~12%) and huge hitbox. Dair can do up to 21% damage if all 3 kicks hit, is a weak spike and sets up for other attacks. You can either shorthop Dair for full damage and launcher or fulljump Dair through the enemy and greet him with an F-Smash at the ground. Nair has virtually no lag when you hit the ground and can be directly linked to anything else, so it leads tu U-Tilt juggled at lwoer percents and actual Smashes at higher ones.
Tilts honestly go to Lucas. U-Tilt can juggle very nice, is fast, great range and KOs @ ~150%. F-Tilt is super-fast, good range, damage and wonderful for spacing, D-Tilt is about as spammable as Ness', does good damage and makes people trip.
Okay, now I'm sorry that I mostly wrote about Lucas, but I've played him a lot more, but I did play with Ness quite a bit as well. I hope I don't sound biased, this is honest opinion. I just don't like it that people try to argue that Lucas can't combo or that his Thunder sucks or something.