I have a RP300 and a GNX3. I use the RP300 for acoustic effects for the piezo out on my RG2027, and the GNX3 for the magnetic out. I've owned the RP300 for about 2 years, and the GNX3 for about a year an a half (got both when they first came out pretty much.) I've used (and sometimes still use when I don't feel like lugging the GNX3 around) the RP300 for my main effects/preamp rig. Here's my thoughts...
RP300 and GNX series both based on the same preamp models and effects. GNX adds amp model warping (subtle, but probably the MOST important distinction) allowing you to mix and match and create custom "amp" combinations (resulting in completely new amp sounds.) The GNX has more amp models, better foot switching, improved functionality, and can hook up to your computer via MIDI or S/PDIF outs (it's got more goodies, too.)
Ok, so soundwise... not much different. The GNX rectifier sounds like the RP300 rectifier. The effects sound just about the same (a TAD better on the GNX.) But when it comes to amp warping... well, I only use custom amp models. The basic models sound pretty good, and to my ear, decently authentic (I.E., as good as any other simulations. You draw your own conclusions there
) But with warping, you can mix and match the "best" of your favorite amps to create great sounding originals. I only use a few amp types, (not really much of an amp sim junky) but they're all custom. That's where the GNX shines.
So, for my money, as someone who has used and continues to use these Digitech goodies extensively, (they're my main sound rigs) I'd go with the GNX. It's just THAT much better than the RP300. But quality-wise, (build, sound, reliability, etc.) they're basically identical. They're not different beasts altogether, more like slightly different flavors of the same machine, with the GNX being a little more "upscale."
Hope this helps.