It's interesting to see that Ibanez had given them different model references because by the looks of it, they are identical. However, the difference lies under that maple top; the RG20126 sports an alder body while the RG8420 is mahogany. Cult Ibanez fans would be a little annoyed not seeing DiMarzios in these top tier models (both guitars featur Duncan's C5 (B) & Jazz (N). I'd reckon they'd wait for other models to come by.
I've tried the RG8420 a few days ago through a tube amp & it's one heck of a player. No, not because it features my preferred pickup brand but because it's a well-made, good-sounding instrument in its own right. I'm not keen on one though due to that vine inlay as it alludes too much to the JEM; I'm not into elaborate fretboard markings so I'd give this one a miss.