Really loud, nascarish sounding = LMIs(imo sound horrible if paired with an o/r pipe, If you stay with a catted pipe it will be 10x better.)
More agressive tone, still pretty loud = LMIIs(my current setup and I love the sound. I have a o/r X pipe also. Still sound real good with catted pipe.
If you want a not so loud sound go with either flowmaster or regular magnaflows.
With SLP, say you got the LMIs, if you don't like the sound you can get either the LM2s or powerflo mufflers and just swap out the mufflers. There all the same size so it makes it real easy.