I am ready to buy an aftermarket exaust!I have a bullydog triple dog.I have heard that 5" is overkill but does it sound better than 4"? Also I am going to run it straight pipe,any help?

I have a 4" straight off the turbo no muffler or kitty and I love the sound of it. I have not heard a 5" on one. Live in a small town so not alot of the trucks around. Oh and it is on a 05 LLY

4", 5"...only she knows the difference (I'm speaking of your truck, of course.) IMHO, the most important issue at hand, which affects the price, is what do you want it made of. You can go from cut and fit of the stock, 409 stainless 3 1/2" on your truck for cheap by replacing the muffler and/or cat with straight pipe replacements. Aftermarket you have the choice of aluminized (will eventually rust, $200-$400), 409 stainless (will soon discolor to a brownish shade, $400-$600) and 304 stainless (stays shiny forever, $600-$900++ ). All price ranges are just round numbers.
After you decide on material, and hence, a price range then other options are sort of mute because a straight pipe is a straight pipe...well, you know.
I have a magnaflow and love it, but, if I were to do it again I would consider an MBRP because they are the easiest to swap out your cat if needed due to common, stock location flanges. Others may too but that's the only one I'm aware of...and I'd buy from one of our supporting vendors! :D