I’m with @Fossegrim here.
In theory, the difference in playability between a good Japanese-made RG550 (or Prestige or J-Custom RG), and a Japanese made JEM is going to be pretty slight. At the prestige or better line you might get a ltitle more attention to the fretwork, but both are going to be great guitars, and a RG with a pickup swap from the V7/V8 set to something a little nicer is a very serious guitar.
That said… if you’re going to be more jazzed to go pick up that JEM off your stand, than a equivalently well made RG, simply because it’s a JEM… then it’s worth the extra money. Buy the one you’re going to play the most.
I have a pair of RGs, a RG550RFR with obnoxious green dimarzios and yellow knobs just to have an obnoxious 80s guitar in the arsenal, and a RG3120TW with a mahogany body and flamed maple top with a very pretty trans teal finish that I kinda intended to sell after buying my mahogany/maple topped Suhr Modern 6 but haven’t gotten around to it just because it really is a very different sounding guitar - darker and smoother than the Suhr, and the other RG as well, and because I just haven’t wanted to deal with it. They’re both awesome guitars. I never really had a love affair with a JEM so I don’t think I’d replace either with one, but you do so your mileage may vary.
I also struggle with the thinner neck, but it’s a lot of fun for short periods of time, and I’m in less than $500 for each guitar, I think, so I don’t mind the fact that they’re not “regular” players for me.