The GRG neck is similar to a Wizard II neck in profile and dimensions but it's only one piece instead of three (and the rule of thumb with necks nowadays is that more is better, ESPECIALLY when you're using a trem). Also, there's definitely a difference in the quality control as you go up the ladder. The higher the series, generally the fewer defects you find in a guitar. But honestly, unless you're buying a
Prestige guitar I would make sure that you actually see the guitar that you're getting BEFORE you buy it.
Now if we're specifically comparing the GRG270DX to the RG370DX, then here are the differences:
Neck: 1 piece GRG neck/3 piece RG neck
Body: Basswood verses basswood (dunno if the quality of the wood is really any better than the other but the guitars are probably made in different places).
Pickups: Powersound verses Infinity (the Infinity pickups are definitely better than the
powersound pickups but they're not super great by any means. If you're going to change pickups anyway then it's not even that big of a concern).
Bridge: ILT trem verses the Edge III trem - the Edge III trem is vastly superior to the ILT trem but if you're going to throw in a FLOYD anyway then that point might be negligible as well.
So really, your best bet would be to get a used RG550 or 570 and then the only thing you'd have to worry about upgrading would be the pickups, not to mention the fact that they are MIJ instead of MII or MIC. It would definitely be cheaper in the long run.