[COLLAPSE="Marth and Peach"]Yay, Marth/Peach.
In terms of MUs:
Marth does better vs Fox (literally trillions and trillions of people play Fox, so this is still important), Falcon (slightly), Puff, and he does better vs Marth than Peach does, imo.
Peach might be slightly better than Marth at Falco, cause she can handle him better on not-FD stages, and Falco HAS to respect her float, so lasers mean less. I actually think Falco edgeguards them both brutally (yeah, Falco's FJ is really really high, and forces her to respect him so much more when she's offstage, and she often has to recover low, to the ledge, which is very bad for her, since she has no ledge options). I actually think that Peach has better combos on Falco than most of the cast, though, that are a lot trickier to DI out of. And again, float.
Peach is slightly better vs Sheik, imo, only because she edgeguards Sheik fairly well (not as well as Sheik edgeguards her, but better than how Marth does it). And I think Marth gets edgeguarded harder, but maybe it's the same. Marth has it easier in the neutral, but the nature of his character and his hitboxes makes it a bit more gruesome for him to get hit/grabbed/touched by Sheik than Peach. It's harder for him to get down. Sheik pretty much poops on both of them, though.
Peach is clearly better vs ICs, and probably Doc too. After that, it doesn't matter.
In terms of character strengths:
There are three BIG things that Peach has that Marth doesn't, imo, which might make the difference.
1) She has a vastly better recovery
2) She has a much better shield/OOS game, which is showing to be VERY important now
3) She has a better combo break/momentum shift (floating out of combos, nairing, tech in place > downsmash)
She also handles shields better than he does, and doesn't suffer as much from being hit into the air.
Over her, Marth has:
1) Range. Discount his range all you want, but this is extremely important when comparing him to a character who has an effective range of zero (she's too slow to do the Fox thing, by compensating with run speed, and she doesn't have Falco's gun, or really long limbs (Falcon, Puff, Sheik). The only range she has comes from a turnip that closes off her really good ground game when pulled, and forces her to commit to an action.
2) Marth boasts a much better ability to abuse positional advantage than she does, which is also showing itself to be more important. Being above Peach and not in hitstun is far less worrying than being above Marth and not in hitstun, due to his huge hitboxes (and not having a terribly DJ that inhibits aerial follows). Remember, I'm saying not in hitstun (Peach uair combos are still really vicious, obviously).
3) Mobility. I don't have too much to say about this, but Marth is very mobile, and in general, Peach isn't. She doesn't boast control of large spaces like he does, with his range/WD/DD, and he techchases/follows a lot better. In a lot of cases, you can just roll away from Peach, and she can't chase you (though you may end up with less stage, which still sucks, ofc). There's no such thing as rolling away as a general option vs Marth.
At this moment, Peach certainly seems stronger, but to be honest, I think he edges her out slightly, still.[/COLLAPSE]