600 are big until you hang with the glow guys that flies 90 size birds. Remember the T-Rex 600 is "only" 50 sized. But they are far more agile than the larger birds.
I agree with the above statements. Size definitely contribute to stability. My 600 can literally hovers hands off. Only downside is the initial cost, specially batteries.
If 600s are too big for your taste, Align is about to introduce a T-Rex 500 by end of February. This will be an in-between size of 450 and 600, similar to a Logo 10. So if you have the patience to wait another month or so, you'll have even more choice (or dilemma).