there are a few things to consider when buying a tuner. will you be getting a custom tune at any point in the future? or are you only going to use emailed tunes? is a power adder in your future? if so, then you should use a custom tune.
to get a custom tune, you will need to go to a dynoshop and have them tune your car as they make pulls on the dyno. this is the time to find out what brand of tuner they use, because most dont use more than one type.
if the local shop uses SCT, get SCT. if they use diablosport, get that one.
with all that being said, i have had good results with my SCT tuner.