A synth is going to actually duplicate the sound of other instruments. Also, you have to alter your guitar with the addition of a synth pickup (which can be double-side-taped, but most people screw them in). I'm sure they do more than just instrument emulation, but I have only played through one a few times.
If you were looking for simply different tones and effects, you'd be better off with a processor (you'll also be hundreds of dollars lower on price). From what I've read, the GNX is supposedly revolutionary. Personally, I'm not a big digital fan. I like a good guitar, scramin' tube amp, and a wah (I'm working on obtaining or making a uni-vibe soon, but that's about it).
All my tone comes from where my volume on my guitar is set. I rarely even change channels on my amp. Oh, I have a reverb unit, too, but that's not a "real" effect. It's required.
Go to a Guitar Center or Sam Ash, etc. and just play with stuff. Take your own guitar and just twist knobs for a few hours. You'll find what you want. Anymore, these stores will match Musician's Friend prices (they may add the shipping, too, but you're still dead even).
Good luck.