As far as I know the only real difference is a little bit more tq in the mid range. I think you see a gain of 20ft lbs around 4500 ish but as far as peak power, I think they are pretty much the same. A lot of people feel that the difference isn't worth $450, but to be honest, it really depends on your goals for the car. If you have the money to spend and you want a nice circuit track car or a stop light to stop light killer, the HTA will probably be a good choice.
I would suggest looking over on
evolutionm.net for more info. It seems there is a lot more users with experience with these turbos vs the DSM scene.