Given that I have an RG7321 and an RG7420 within arm's reach, I feel I can legitimately comment on this.
While both guitars are not perfect out of the box (the pickups suck on both) the rg7420 has everything you'd expect from a japanese ibanez. The neck is far better, is more comfortable due to the rounder, thinner profile.
The RG7321 however has terrible hardware (tuners mainly), a cheap soft plastic nut which hampers tuning stability and has the wizard II neck profile, which has the 'shoulders' that people often complain about.
Also don't be all that put off by the 7420's trem. I don't use it all that much but I've never had any tuning problems with mine.