Ive had recent experience w/ 4in mbrp w/ and w/out muffler on my truck, and I went to the muffler shop w/ a friend who got his stock 01 straitpiped. Hands down, mbrp sounds alot better straight piped compared to having the muffler on. I didnt think it would be a big difference either but I was really wrong. The turbo sounds really good and honestly I dont really envy the sound of CTDs anymore after I went straightpipe. As for difference between stock 3.5 and 4in straight, I think the 4in has a deaper resonance while the 3.5 seems to be raspier(reminds me of glasspacks on a gas truck sorta)