My man.
Peach>Marth would be an atrocity.
I just can't reconcile the amount of dead space Marth can control with his sword proximity. Sheik actually has a projectile, has a reliable move/set-up to fish for kills with, and I still don't think that's enough to compensate. Unfortunately, you'll never see this effective range in action because Marth's have a ****ing stagnant metagame based entirely around poorly copying a player from half a decade ago.
Marth wins in terms of match-up comparison.
Marth fares better against: Fox, Falco, Puff (I think), Peach, Ice Climbers.
Sheik fares better against: Head-to-head, Falcon.
Past that tier, they both beat everyone. There's a prevailing attitude that Sheik ***** the (of little significance) lower echelon harder, though I think that ties into my earlier point of Marth's being stupid and not effectively realising they have a ****offenormous sword to hit bad characters with.