im planning a build that will get around 400 to the wheels and i'm looking for a decent ECU that won't break the bank.

 

An Apexi PowerFC is going to be the best bang for your buck. Not everyone is a fan of them, but you can get perfectly good used ones here for $500-600. They can control all of the factory equipment and the handheld "Commander" is extremely helpful for monitoring and tweaking.
Ultimately, the decision should come down to what your tuner wants you to get. Many have a preference and it will be hugely beneficial for your car if your tuner knows the ins and outs of whatever ECU you're using.

 

Another vote for the PowerFC here. There are certainly "better" ECUs nowadays, with more advanced safety features and controls built-in, but I like that the PFC has been pretty much figured out. There's a wealth of information about it on this site and others, and there are tons of them on the street.

That said, I'm definitely biased, and I'm a sucker for the older Japanese tuner parts/kits. "Best" is probably a measure of what you or your local tuner can make the most of when it comes time to dial-in the car.

 

Find a tuner you trust that's experienced WITH A ROTARY. Ask him which one he prefers since he's going to be doing the tuning.