awh, dude just get what you like and feel comfortable on.
600, unless you are a track racer, you're not going to out ride that bike. great for canyons. just lacking on kick for straight high speed runs, yet just a few seconds to get to high speeds than 1000cc.
750 overall, a great all around bike, stunt, canyons and every day riding with some power to it.
1000 good bike, has the kick, stuntworthy, as well, new bikes have better gearing for canyon riding. yet, don't need the power in the canyons.
any bike, does not matter, you control the power and can learn on it. so, you get what you can afford and feel comfortable on.