I'd say the 550. In general, a JEM is an RG550 plus a monkey grip and a nicer output jack location.
EDIT: Plus better electronics
EDIT 2: Plus better trem routing, different inlay, and nicer finish, is that all? rofl
EDIT: Plus better electronics
EDIT 2: Plus better trem routing, different inlay, and nicer finish, is that all? rofl