Which do you prefer? Button lock-on or Auto lock-on?

I prefer button lock-on because it's more reliable.

i like button lock on. i don't like auto lock on because it kind of gives away places that people are hiding. when u walk by the gun will just go zing onto the person that is hiding.

Anybody else have an opinion on this subject?

I've never played a game where auto lock-on has worked reliably, at least no more reliable than a button lock-on system would be. Those WWE Smackdown games- that immediately comes to mind when I think "crappy auto lock-on system".
 
The Legend of Zelda: Ocarina of Time's Z-targeting is the pinnacle of button lock-on systems. It will never get better, and has not gotten any better in any game since.

If I am playing some game like HAWX, auto. But everything else is button.

Button Lock on unless it's like...a flying game. Or a section of a game with flying.

Button lock on for the most part.