Hi Scott,
Good choice, I love Eheim pumps! I personally would lean toward the 1260 just because I feel like the 1250 would not be enough. If you think you have too much flow coming out of it you can always dial it back a notch with an inline ball valve, which has no adverse effects on the pump itself.
Also, a lot plays in to how you are have the returns plumbed in the tank. 600gph with just one nozzle that is static going back into the tank is a lot more than 600gph going through a rotating return, or even through two returns.
Thanks,
Jeremy