They're all designed for guitar players. The 1000 is actually an upgraded or "progressed" design, though. It's supposed to sound a little better, and in my experience it does, sort of. They are both top notch and almost the same. The extra headroom of the 1000 is worthwhile, though, and in my opinion, that's why it might be slightly better for FRFR. Depending on how loud you play, especially live, the 800 non-bridged (at 400 watts) is kind of borderline. I remember feeling like I was pushing it a little hard to get it loud enough. The 1000 in mono (500w) is perfect and never feels strained. The 1000w model is also what they choose to use in their powered FRFR cabs and wedges (well, they use a GM50 module, which is just one side of a GT1000FX).
You can't go wrong with either, but I think the extra little power on the 1000 is actually noticeable and worthwhile. They're both (the entire Matrix guitar specific line, actually) designed for guitarists, and mostly with the Axe FX in mind, and the GT800FX was their first amp, whereas the GT1000FX was the product of more experience and perfection to make it sound even a little better, though they're both almost the same, IMO, headroom aside.