Turbo S10 Xtreme said:
The horsepower will be the same it's just how loud you want it.
Technically, that's probably true, as you're just replacing the muffler, but I would still get the Borla if you can. Flowmaster makes some of the worst flowing aftermarket mufflers out there. Honestly, I don't like the way they sound on small displacement engines, either.
Sell it to your parents this way: The Borla is a stainless muffler, just like the rest of the stock system. If you just put an aluminized Flowmaster on there you'll be replacing mufflers every couple of years. It may or may not be true, but it's a good selling point.
If you are absolutely dead set on a Flowmaster, I would get the 50 series. It will be a bit mellower and will not resonate in the cab as badly as the two-chamber. Plus, the case length is longer than on the 40 series. You need a 19 inch long, offset in/center out muffler to bolt it up to the stock pipes without doing a lot of cutting and welding.
-Chris