1) decide whether you want it to be quiet, sporty, loud, really loud
2) decide how much you want to spend
3) ask at least four places
I had a 2.5" cat-back system on my VP with only one muffler, by SuperCat, and I absolutely loved it. It was what I'd call 'loud' though. With two mufflers, you'll get a sporty sound from Redback, but it won't be garishly loud. And with two mufflers and a hotdog, it'll be quiet