all downpipes are pretty much the same. only difference is the mouth and if its catted or not. i have had a turboXS, helix, and now an invidia. all 3 were no different. all 3 fit fine. i did switch catbacks on my invidia and catback doesnt fit 100% , but i think its due more to being rear ended and its not 100% fixed where it should be.
i say go with the cheapest you can find. sound wise isnt gonna change unless its catted or not, sound depends on the muffler you choose.