Listening to those video's, the SLS sounds better....more raspy, less muffled. Interestingly though, in real life the GT sounds much more aggressive than that with loud pops and backfires so I suspect in that particular video the GT's exhaust was in it's quiet(er) mode.
I would still say that revving at a standstill the SLS has the edge, but once on the move the GT turns the tables - driving the GT, I thought it was louder than the SLS (certainly from inside) and had a lot more burbles, pops and bangs than the SLS.
But it's like comparing winning €10m or £10m....you're probably going to marginally prefer one, but you'd absolutely love either option!