I agree with the OP - Marriott's fitness centers are one of several reasons I have started moving my stays over to starwood. I have noticed Westin hotels to have far superior fitness centers (not just LifeFitness treadmills, but newer models than most Marriotts have). Even in some of the Marriott fitness centers that have newer machines, they are not kept up well (half the machines are broken, hand weights are missing, no towels, etc.)

I have stayed in far fewer Westins than I have Marriotts, so maybe I have just been lucky so far, but for now I will continue my trend.