6 and 2/3's really.
The old RG550's and Jems are so similar its unreal, that said, I prefer the
Jem neck to my RG550's feels thinner.
Its not really a case of "Which is best", I would suggest going to a shop and trying both necks out then make your own decision, i wouldn't personally go on the reccomendations of a load of guitarists on t'internet. You are the one that's going to be playing it, only your fingers can decide.