Concave all day. Every single one of my yoyos has a concave (or at least a string-centering bearing) in it lest my Canvas; that puppy runs a standard 8 ball.
I like them because they add a bit of unresponsiveness and a little more predictability.
I’m curious to see whether anyone’s done an actual test to see if this is true. In my personal experiences, I haven’t noticed any difference in spin time.
But, in general, I like flat bearings. Concaves just bunch up my string when I don’t want them to, and don’t bind when I need them too.
I only use flat bearings. If a yoyo I purchase comes with a concave or grooved bearing, I switch it out right away for a One Drop 10 ball bearing with a tiny dab of lube.
Something about the feel of a flat bearing keeps me using them. I can always tell immediately by feel if a throw has a concave bearing in it and I always wind up wishing I could swap the bearing.