I'd get the 371. That's what I have. I do tow, and you can tow with the 371 just fine. Just rev a hair higher on hills (either downshift or speed up
) to keep the turbo lit.
With the S371 and 60% over injectors, I get a hair better highway mileage than I got before (about 0.5 MPG better), but worse in town (about 1-2 worse, depending on my right foot). Towing mileage is similar to stock, and a bit worse on hills. I'm not sure if the turbo change or exhaust change is what helped my highway mileage, maybe a bit of both.
Personally, I would skip the 45% injectors and go for 60% or so. I have Industrial Injection 60% overs, and despite all the bad people like to say about II, they are going well so far with ~10k miles on them. Idles fine, and runs fairly clean once I dialed in the tune. I kind of wish I had gone bigger on the injectors, actually.
However, you say a single turbo is your final performance goal, and if you are only shooting for ~600 HP, 45% injectors might be the best bet for you.