Next time please refrain from making a new thread. We have an rFAQ right on the first page for this type of stuff.
http://www.smashboards.com/showthread.php?t=201103
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To answer your question, they are both just as reliable and will connect each time if done correctly. They also both come out at the same time (hitbox actives on frame 5 for both) so they're just as quick. Here's a more indepth look at each move.
Damage given --
Uair ( 4-9% / 2 hits , first hit 4 , both hits 9)
Fair (6-14% / depending on # of hits , 14% being all hits)
Both Activate on Frame -- 5
Uair is ideal for juggling because it keeps your opponent above you in the air, leaving themselves vulnerable to follow up aerials from underneath, most likely more Uairs.
Fair is ideal for getting quick damage and sending your opponent away from you or/and sending them offstage to put yourself in a safer and more desirable position.
Uair tends to be the most common move OoS (Out of Spindash ;P) simply because juggling is a great way of racking up damage quickly, and Fair is generally used as a punisher since it covers Sonic in multiple hitboxes.
--credit to Dnes/djb/SL for the data
