I have all of three types of sound decoders discussed here: QSI, Soundtraxx, and Digitrax. Of the 3, I prefer Soundtraxx the most.
Each has benefits: QSI and Soundtraxx can be modified and programmed if you have their programmer. The sounds of stock QSI's to me sound a bit primative on some of the Atlas' Gold models, although I own only one. I have a QSI Universal in a P42, however, that I think is outstanding. I cant vouch for the new QSI Titan's but some folks on the Atlas Message Boards are disappointed it seems. But that group it seems to me is very hard to please.
The only have two Digitrax sound decoders with 1 being their low-cost offering the SDH164 decoder and a D165AO with a Soundbug that a friend uploaded for me. Again, the ability to upload custom sounds are a plus.
The Soundtraxx to me, though is currently the best. The sounds are clear, and to me, accurate. You cant modify them, and Ive never ever felt the need to wish I could. The amount of tinkering that can be done with the CVs is enough to keep you busy til Rapture. I do wish that you could certain prime movers and horns in different formats aka the SD70M2/ACe in the Atlas Format instead of the TSU format. I also wish some of their decoders came with more than 4 lighting functions.
Brad