Before I say anything, why would you discuss zelda and sheik separately especially for something like this? To do so is to assume that sheik chooses to just cut out her upB from her moveset and that makes no sense when we're talking about who is better considering all of their options.
Sheik is generally known to have bad priority. Her range is decent (ftilt and bair), but not impressive. What's more is important is that she has the fastest attack speed in the game and she's fairly quick on top of that. I rarely if ever see sheik's placing. If you want to see where a good sheik places in large tournies I'd say look at light's results or better yet......
http://www.youtube.com/watch?v=xQ6_0AOy2iw&feature=related Okay this isn't realistic, but I had to post it.
I feel like sheik should be higher than where she is right now so I have to say I think that sonic is definitely lower. Sheik has needles which make for an insanely good punishment tool (why does everyone keep saying camping?).
Her ftilt is more than just a tool to lock, it combos into just about everything including kill moves and I mean TRUE combos. If a really good sheik could harness decay and ftilt well killing would not be a huge problem anymore since she has a true ftilt to upsmash combo on every character and if ftilt is properly decayed and upsmash is not this can kill at percents like 80-100 depending on weight, di, etc. And this is insane considering her racking abilities. She's really good at punishing slight mistakes with lots of damage over a very short period of time.
As for how she does against the top tier. I feel like she does okay, especially against mk as compared to the majority of the cast. She has a guaranteed kill on him at like 80 percent from grab (I have no idea how hard it is to grab a good mk =/) and she can rack decently well on him. She's one of the few characters that can keep up with his speed and mobility. I think that matchup is pretty close of both players are really good. She does okay against snake, but not too good. Does well against DDD and can just switch to zelda anyway. I think she beats falco. I think with proper chain control she beats game&watch.
In terms of people who give her problems some of them can be handled by switching to zelda (well <3 says the matchup's not that bad anyway, but I'm waiting for vids). Marth beats both of them pretty bad. Olimar too. There certainly are no unwinnable matchups between the two of them.
edit: Whether or not the sonic boards believe this, for a low tier sonic is popular. In general, sheik is severely underplayed. Its really unfair to compare tourny results.
And stop comparing marth's fair to sheik's fair. Its mean. Marth's fair ***** sheik's no matter how fast it is. Range is relevant. lol
edit2: Ankoku and Villi both use zelda and ryoko too if he counts. And I'm pretty sure everyone but the most prideful plays zelda if they have to fight ICs.