I think Sheik vs Sonic is 55/45 in Sheik favor maybe 50/50 but it really depends on the stage.
Keep in mind everyone with platforms that sheik can only weakly force approach against sonic and with his good speed the range in which it's unsafe to throw then if they miss is much larger and that you will rarely land the Ftilt against sonic as good sonics typically wont land on the ground helpless. Usually when punishing sonics I find that nearly all of which happen in the air as on the ground I almost always throw needles. Although Sheik has good OoS options and can counter all of sonics mix up aerial approaches but because you lose your Ftilt you lose sheik greatest feature, her combos.
The reason why for so long I've though sheik had a very good advantage against sonic was because on stages like BF she can chain camp him very safely but a good sonic will stage strike BF and then ban it if he's smart. FD isn't smart for obviously reasons, Yoshi island they can bypass the chain using the curves on the sides to break though the chain with invulnerability kinda like DK up B, Lylat Cruise I fear could have that happen as well and smashville is just like FD but with a platform.
So really we don't have any stage advantage on the first match but neither does he. Let's say we lose where do we counter pick? Well my first thought was chain camp but BF is banned so what stage to pick.
Halberd is most likely the best choice because of it's low ceiling and long platform but roughly half the time he can beat it by going below the stage and attack from below.
Pokemon Stadium platforms don't last very long and aren't very large which make poor shields. On some parts you can camp fairly well don't get me wrong but you can't stall by camping so why wouldn't he just wait?
Delphino Plaza pretty much is a mixture of the weakness of Pokemon Stadium and Halberd, enough said.
Castle Siege, Frigate Orpheon are very good stage and has some very good platforms but they don't last forever.
Rainbow Cruise could possible be counter pick against sheik for sonic but meh.
Basically sheik can't chain camp sonic very well.