If you're really going to build up a drag car I would suggest the STi just for the 6MT alone. The transmission in the STi alone is almost worth the price premium on the STi. The .5 additional displacement is nice to start with but nothing else much on the STi is going to be helpful over the WRX if you're building it up for a seriously fast drag vehicle.

I still would agree with everyone else that the Impreza isn't the best drag vehicle choice.