Sheik is becoming my favorite character to deal with Marth (I'm too stubborn to go MK
) and I'm going to agree that it is relatively even, but Marth's range is going to give him a slight edge. Though once Sheik finds momentum, I would say things shift in her favor as it always would. My Sheik is still new, but when I play Marth I just keep high pressure on him at all times. Tippers = bad, so if he doesn't have the room to space correctly then that's one less thing to worry about in my book.
Marth is going to have to constantly space aerials to keep Sheik away because he is combo/gimp bait for her. As soon as Marth leaves himself open for a split second Sheik is definitely quick enough to take advantage of it and start some hard punishment. Landing a jab (to jc grab), Ftilt, grab, Fair, Nair, the list goes on as to what can start ****...
It's not hard for Sheik to carry him offstage, and once he is off he is fairly easy to gimp. I find myself trying to hit him with needles as he double jumps to cancel that out, and without his jump he is forced to Up B and that's very easy for Sheik to gimp. You can hug the edge at the last second, but its easier to just hang off with the chain and not even have to worry about putting your body there, and you can always instantly get the invincibility frames if you ever need to. If he can get back safely, then I try to go for a Bair or Fair to send him back out where he is hopefully now without a double jump.
The biggest things Sheik needs to worry about are obvious. More range is the best thing Marth has to abuse here, and plenty of power on his tippers. Needles, fast shffl aerials, and tilts that lead into a devastating combo game are all going to keep pressure on Marth where they will have less time to think and become more prone to a mistake. I find that giving Marth any room to space his aerials is just what they need to play a good game of keep away, and Marth definitely wants to keep Sheik away.
We all know how good Sheik's pressure game is, and I have had the most success abusing that and not giving him room to breath. If Marth can't land tippers then there goes his power advantage as well. Marth also has a good edgeguarding game, but mixing it up between Up B and the chain makes his life so much harder and leaves Sheik living that much longer. Once you find the momentum it just unfolds into a pretty easy gimp. If Marth gets the momentum then Sheik is going to have a harder time finding an entrance and allowing Marth to maximize his range and power is bad.