well i've heard by experience that the LS1's (therefore LS2's), have a much lower, throatier rumble than do the little 4.6L's. While an LS1 with a free-flowing exhaust will be noticable at idle, and only get louder and meaner through the rpm range, mustangs tend to be barely noticable at idle, with a noticably raspy yell when the throttle is goosed.
Generally, it's easier to make a mustang sound better than an LSX series motor. I'd be willing to accept a pickier catback option however for the extra 100 horses. :cool