Being a new member to this forum and just recently contributing here, I’ve read different opinions about which bearings are the best and the answer will probably be debated forever and ever, just like which cars are the best, or if pepsi beats coke, etc etc. Players who have them obviously already know what they look like but for those who haven’t tried out the concave type, I decided to photograph all three for you to look at so you can see that there really is a difference between the CBC central trac and the Difeo Konkave bearing.
The first bearing starting on the left IS the CBC Central Trac. As you will see, there is a completely flat center to it with angled edges. The second bearing is a YYJam Standard flat, and the third bearing is the Difeo KK. And as you will see that one has a completely smooth shallow U shape, without the flat bottom as in the CBC.
In my experience, both concave designs work nice, are long spinning, provide great balance in keeping your string away from the yoyo walls. The KK tends to overlap the string a little more because of the U shape and the CBC allows more width in space for the string to overlap because of its flat bottom. This is more noticeable with the use of thin string.
On all my yoyos the concave helps me with both balance and extra spin time, say around 20 more seconds on average. Mainly because flat bearings provide less balance than the concave ones do being that the string moves from side to side etc. So when I put the concave type in, problem resolved and with a bit more unresponsiveness too.
Ultimately direct experience and personal preference is the answer, However I do hope this pic comparison can help.