usualy people use 0.25s in there guns due to increased accuracy and range. if ur planing on using them get a good brand like bb bastard. also don't forget to adjust ur hopup but thats a given. the difference between black and white bb's is that the black bb's are extreamly hard to trace wich means u don't know where ur shooting. White bb's are easy to trace and some players that have a good eye can trace them aswell wich means they can hide before it comes near them. Alot of people like to mix both because if your gun is fairly accurate you can trace with white and people won't find out your position as much.
hope this clears things up