well my experience of megasquirt is ok, if i had the money i would have gone dta and got someone else to map it and save me loads of hassle, but a great learning experience.
all those ecu's will give you good power, it's all the transients that are the deal breakers and no reason to avoid any.
400-450bhp on a zetec turbo does require a fair amount of money though, quite a few hit 350-400 and are drivable. if you feel confident in your mapping skills go for it, i go safe and tune out towards lean to get it where i wanted it, just waiting for a better and bigger turbo now to push on again, hit peak of 29psi and held 26 with current turbo but started smoking.
ultimately go for what you want to work with, megasquirt can be as good as any other if it's been built properly.