I've used the Roland GK and guitar synths for years. I think you're better off with the Roland system; if you want a guitar synth, look at the SY-300 - you don't need a GK pickup; if you want more capabilities for guitar / stringed instruments the SY-1000 is worth a look, and you'll need a GK-3 pickup.
If you want to do guitar controlled synths, that would be the new GK-5 pickup and GM-800 - which will give you more keys, synths, and sounds similar to a keyboard based synth.
I own the SY-300 and SY-1000; since I have synths, organs, pianos, etc., I just use keyboards, so the GK-5 + GM-800 aren't needed.
and now with MPE keyboards and software, I really don't need a guitar for expressiveness, as I have keyboards that can do that now...and expect more keyboards to support MPE, poly aftertouch, etc.