Different brands use different size bushings/bearings. Even within some manufacturers, similar models use different size bushings/bearings in gen1, gen2, gen3, etc... (i.e. first G&G M4's might have used 6mm...then 7mm in the next revision...then 8mm..)
I'm not 100% sure on this...but I think there have been mechboxes where 4 of the 6 bushings were one size and 2 of them were another. Really not sure on that...I might be completely wrong.
At any rate, IMO the general thought is a solid metal bushing is more robust than bearings or a plastic bushing. A lot of bearings are now 7mm and 8mm (and I think there's a 9mm as well). The smaller 6mm bearings had a bad rep of not holding up with a lot of use. Wear/tear and heat build up are killers. Bearings do run smooth though.
Now...I personally don't see the point of bigger and bigger solid metal bushings...other than to replace larger bearings.
Most importantly...buy good ones and put them into a solid good mechbox shell. There's no point in buying cheap bushings and putting them into a cheap shell...just to have them rattle around (I've seen plenty where the bushings didn't fit tight in the shell and you could actually see it move around in the hole).
Also, with buying cheap ones you have no idea how they will stand up to wear. Metals can be soft or hard...and you definitely want precisely sized hard metal bushings....otherwise don't bother.
Best of luck,