I use stainless all the way. In salt water with no suit I still need about 8 pounds over my plate, so stainless just makes sense. In fresh water and no suit, I'm just fine with the plate and no weight at all. For me, since I always need at least seven pounds, stainless just made sense. It takes more weight off the belt, and has improved my trim. I prefer to keep as much weight as possible on my rig, and keep my belt to about ten pounds max. I don't mind the weight on my back.
My wife on the other hand is quite happy with an aluminum plate for the opposite reasons. She uses 6 pounds over and above in warm salt water, but doesn't want to make her rig any heavier. It's already cumbersome for her. So she prefers the weight on the belt.