Howdy Sonic Boards! Let's get this under way.
On the ground.
On the ground, there is not any advantage. They have similar moves. Luigi is slower and floaty, while Sonic is solid and fast. Luigi has more K.O moves here and there. Your homing attack in this matchup is not a good idea on the ground. The reason Luigi has the advantage is because:
1.) K.O opportunities
2.) Lighter combo's like a SHNair, and u-tilts, (juggling)
3.) Approaches with projectile, Sonic lacks a projectile, so a fireball setup works.
I'd say for Sonic advice on the ground: Be aggressive, because quick jab> grab> dthrow> aerial will go a long way for you. When approaching, make your move as quick as possible when Luigi is even slightly vulnerable. That's why you want to say close but not too close. Luigi can Nair his way out of any closeness problem. Again, I think don't go with homing attack in this match up because Luigi has his f-smash that will take you down quick if you miss or bounce off a powershield.
Luigi's u-smash should be used a lot in this match-up. Using an OOS u-smash after a powershielded f-smash or homing attack will get you killed. Take advantage of Luigi's sliding by anticipating when he starts to slide. If Luigi tries to u-tilt juggle you SHFair, you'll get him. It is harder to UpB Sonic because he is so much faster, Sonic would need to be too close and vulnerable. To avoid Luigi's constant SH approaches you need to spam u-tilts as a Sonic.
Ground 50-50
On the edge this game turns around. Sonic has a predictable recovery, but it is not in the same area ad other recoveries, it is higher up if you used UpB. Luigi would have to mix up his recoveries well, because if
Sonic were to sit and spam his f-tilt on the edge stopping green missiles and tornado recoveries, Sonic would win. Sonic can intercept Luigi for higher damaging, but Luigi can intercept Sonic for the K.O, Luigi has a bair SS and a dair spike, which are hard and situational, but if you are predictable a good Luigi will get you. Luigi's d-taunt works the same on everybody, if they don't think fast, and try to come for above, a well timed Luigi d-taunt will send you offstage, or in other cases bounce you off the ground spinning where you scream "UpB me!". A semi-common Luigi edge guard is to UpB right when you can touch them, but Sonic can attack after UpB and protect himself with fair. Luigi's recovery is hard to gimp, but not hard to see what comes next.
Edge: 55-45 Luigi. Reason: Combos
In the air. Luigi and Sonic are both aerial aces. Sonic has ridiculous priority with his fair, and that stops Luigi from aggressively applying aerial pressure. Luigi may have more combos to choose from in the air, but Sonic can rack up the damage just as fast. When I play Sonic I find myself falling back constantly and spamming bairs. I just played a second ago and noticed that I didn't use dair at all. Which isn't bad because it is too short range to work against Sonic. Luigi can assure a safe descent with fireballs and bairs that Sonic can't so in the air, you need to be close as well.
Aerial: 55-45 Luigi. Reason: Combos
Taunts:
Luigi's taunts can hurt you
Taunts: 100-0 Luigi: Reason, hurt
In the end, I think this match-up is very close. I think, they balance their weaknesses out with strengths.
Luigi has a slight upper hand. Luigi has a 61-39 Advantage overall.
Luigi can just rack up damage a little faster, and then KO for the win.
Sonic General Strat: Don't let Luigi mindgame you and change up the approaches. Anticipate where moves like UpB and tornado leave him vulnerable and protect yourself with tilts. In the air, bairs, and fair will help you win. Stick close to Luigi because you have no projectile game or aerial range. Homing attack is a no-no.
Luigi General Strat: Do what I told the Sonic's to avoid, but keep them on their toes.
If Luigi can combo Sonic into the air, that'll keep him honest and not try to pull off anything drastic like running in and spamming K.O moves.
I think that covers it
EDIT: I almost forgot the stages!
Luigi would pick a place that's simple aerial and has platforms. Sonic will get beat in BF and Lylat Cruise.
As Sonic I would pick Brinstar because it can go either way, and the blast zones are big and wide. I don't know if you guys like that course or not. Also you might want to do Delfino for the movement and maybe even pictochat. Pictochat gets in the way of some of Luigi's approaches.
Don't choose Corneria or Luigi will win.