There is a world of difference between a 600 and a 1000. Not to say the 1000 is better, or the 600. I personally do not like 600's, highly recomend a 600 to anyone else. The 600's are light, nimble and fun to flick around on twisty roads. The 1000's feel bigger and bulkier, and accelerate like crazy. Kind of like comparing the 450r quads to the 700's. The 450r's are light and nimble, the 700's are bulkier, but pull harder. There are lots of ppl that have owned the bigger bikes, and preffer the 600's. As for top speed, any 600 or bigger sportbike will go way faster than anyone should go. If you know someone that will let you test ride one, it can help a lot with choosing. If not, go with a 600.
As for brand, personal choice, but the more poular a particular model is, the more aftermarket support there will be, and the easier it will be to get replacement parts if needed. (cbr600, zx-6r, r6, gsxr600)