Well these are two of the more underrated middle infield prospects there are in the minors. Aybar is over shawdoed by Wood/Kendrick and Sanders cause well he has only performed in Low A ... but I like the kid and he is starting to get more attention now.
Anyways to the question, overall I like Sanders to eventually be better then Aybar. I think Sanders ability to take walks will help him greatly as he gets to the majors, even though he K's a good amount, it shows he has a good eye at such a young age. Also though both steal, Sanders is already superior to Aybar in that cat. as he knows how to use his speed. It also doesn't hurt that Marcus should play 2B and Aybar SS, just adds to Sanders side a bit more.
That being said, Aybar will most likely have the better line up around him with the Angels spending money and have great farm system depth. It makes it a bit more interesting, but that might be getting a bit to far ahead of myself.