I was kind of wondering the same thing, its not a problem to me, but I noticed that recently too. I play my PS3 often, and leave it on for hours at times, figured it could be because of that.
Haven't played GRAW yet, but R6Vegas is sweet. Vegas is an overall fun game, although for me playing it online was a down fall, seen as everytime i wanted to do so, it lagged. Cant really compare them, but I'd still recommend anyone to get R6 Vegas.