I went through this last year when I re-did my exhaust. I was looking at 2.5 or 3" and Aluminized or Stainless...I decided with 2.5 Aluminized. Not because of cost factor but because the sound of the pipe and clearance under the car.

I was quoted (converted from Euros) $650 for 2.5" Stainless. The problem with the stainless is that the wall thickness is super thin so it is going to resonate much more.

I put 1/2" Stainless flanges on my exhaust because I may want to disconnect portions down the line to remove the trans lets say. Why Aluminized and not SS? Who is going to crawl under my car to see something I paid so much for? I had 12 yrs on the last system and it didnt oxidize or discolor.

I think 3" is overkill. Thats just me....I have 2 1/2" pipes into Flowmaster 70 series mufflers (Diesel).

What will you run H or X pipe? That was what I battled more with than anything on the work....I went with an H but I also installed exhaust cutouts and it SURE IS FUN!!