The other thing to keep in mind is replacement costs. The cost of blades on a 600 and bigger get to be a real issue. While Gaba brings up some valid points about thrust-to-weight and overall efficiency of the 550 vs. 600, with increasing mass comes increasing momentum.
I have dumb-thumbed my 250 into the dirt while diving out of stall turns and gotten back into the air in under an hour. If you do that with a 600 or bigger, the momentum from the extra mass is going to break a lot more stuff. Forget about replacing just the main blades, feathering shaft, and maybe the flybar and tail boom and start thinking about motor mounts, carbon fiber frame components, and battery packs.
I am looking at something bigger, but the 450 and 500s seem to be more attractive to me. I think that I am with Tony in the fact that 700s and up scare the hell out of me.