The gbbr market is still pretty new...not so long ago, WA released their gbbr system, but it had loots of problem and need at least 300-1k$ worth of internal upgrade to works flawlessly.
Then WE came out with their own system wich some call more reliable....( maybe?!) , but one problem still remain with those 2 platforms... The cooldown factor/effect, wich reduced fps and ROF (and may also be responsable for parts breaking....( metal doesnt like cold... ( at least the lower quality metal in chinese guns... :S).
And now KJW was able to limit the cooldown effect, with another system....
Next VFC is supposed to release their own Gbbr line too! (SCAR, SPR, M4!)
So I would wait at least a year before buying a gbbr, now we have too many "beta" gbbr on the market, nothing 100% reliable yet.... So I would wait and watch them evolve!