Both are good companies.
Flowmaster has been in business for a long time and they are known for being in v8's
I say go with flowmaster. I put a 40 in place of the stock muffler and left the exhaust the same stock size, no duals and you would not believe how it sounded. You would have thought I put duals on it.

 

Im not really so much worried about the loudness. I just like the sporty sound of Borla. Its like a purrrrrr, until the revs get higher. Then its just sexy.

 

I just had a flowmaster 40 put on my truck and a guy that works with me said that flowmaster is the worst because it has to much back pressure....has anyone heard that befor??

 

Nope never heard that. Flowmaster is a aftermarket performance muffler. It is going to be better than stock is. Also don't be scared to start a new thread instead of pulling up an old thread.