One really isn't any better than the other. Both stuff air down it's throat - both will improve the performance (neither one in my opinion over the other) if looking at buying new systems I would recomend the Banks first only because it comes with everything you need front to rear (exhaust, gauges and trans controll for the automatic guys) the ATS is cheaper up front but once you add in the above included options the price is the same.
Now the hypermax as EZ mentioned is a good one as well but again no options in the box - it still comes out with the same level of performance it's just that it's not wastegated so you will see a little bit of turbo lag with it compared to the others - not much though... Not enough to keep me from buying one.