Physically there isn't all that much difference between the RG550 and the FP. The FPs had the lion claw but then the RG550s have decent enough routing to accomodate the trem. The
neck shape will be slightly different and the pickups on the RG550 (unless you've changed them or they've been changed) will probably sound a little different; the FPs came equipped with Dimarzio PAFs. The trem is different I believe as well, but both are fine and some folks prefer one over the other and they're very much viewed as equal (a rarity for anything) within the community.
I'm going through the same thing. I bough an RG550 about a year ago used and immediately it became my number 1 until i started playing 7's, and then I got a Universe. I'm playing the RG now and again and have liquidated alot of my other guitars so replacing the RG with a guitar I've always wanted (a Jem) has come to mind. However every Jem I pick up and play has been less than a positive experience. I typically find the new ones are set up terribly. Used ones are occasionally even worse and have usually hadd the snot beat out of them. For the money I'm just not that impressed with what I've laid hands on. I got a great deal on the RG550 ($250....it was a guitar that was bought in pieces by the shop, re-assembled with new repalcing the missing parts and mis-marked when put back on the rack) and am of the conclusion that for under $400 total I could have a guitar with just about the same specs as the Jem aside from minor things like neck shape, trem style, etc.
Sure, it's not as prestigious as a signature axe....but it's a great player and that's what I really need. I use my UV as my main and a 6 is just kind of a fun little diversion to me.
...hope youre having better luck finding them setup than I have.
