Get to the dealership on a weekend or hit some local events and ask other riders to fire their bike up so you can listen to them to see what appeals to you. I would not punch out the stock exhaust, but, that's just me. There are a pile of choices. May depend on what else you have planned for it. Are you gonna decat the head pipe, any cams, free flow a/c, tuner, how much you want to spend.....?
Ericj, I just bought a new SGS and put on V&H Twin Slash Cut slip ons. I am very happy with the sound. They are much louder than stock but not obnoxious. They give you just enough sound while in the seat where you can hear them in the background but don't drown out the radio when going down the road. My wife drove behind me in a car the other day and told me they sounded great "off" the bike. The V&H have a nice deep sound and to me sound like a Harley should.