Re: Jem vs RG3120
To be honest I think it boils down to a three things
The guitars are different woods, alder for the JEM and mahogany/maple for the RG they have different pickups that (whilst easily changed) accent the differences in the sounds of those woods.
With regards to the looks, one is a brigth metallic blue with a handle and a vine inlay, the other a flame top, two very different styles, which do you like the look of better, the pseudo lake placid blue or teh pseudo-gibson flame top... If you gig regularly which suits you and the style of music you play more, the bright blue or the more "serious" flame top.
With regards to the playability, is there a difference in hardware, I'd always for go for the Lo Pro Edge trem over the Edge Pro, that's my own personal preference. the JEM neck would be more comfortable over long sets and practise sessions, but the Wizard neck might give you slightly more speed in the short term.