There is a related topic on different types of concave bearings
My strong pref is for a fairly strong concave bearing, because I suck, and my throws on a flat bearing may end up on the far right or left sides dragging against the response pads. The counter argument to that is when you do lots of string layering tricks, a flat bearing gives more room for the string to naturally spread out more.
I agree with @ferrocile that the “groove” or “V” type is a little too extreme, as the string is basically forced into the exact center at all times, so perhaps the best compromise is the “pixel” or “double-straight” type with multiple steps?
That or pure concave would be my pick.
I do not feel ceramic bearings are worth it at all, because they’re noisier, more fragile, and don’t really offer that much practical improvement over a quality stainless bearing… and they are all flat too. Well I guess the ceramic ball type could have concave stainless outer ring, I suppose, but the full ceramic ones that I’ve seen / used are always flat.