The 9100 is basically the 8500 chip, but a lot of 9100 cards are only running 250/250MHz for core and memory clock speed. The 9000 Pro and 8500 (non LE) run 275/275 and some 9100 cards are also, just check before you buy.
A 9000 Pro vs. a 8500LE/9100LE will be a very very tight comparison and although the 8500LE/9100LE may well come out on top generally, the 9000 Pro will have a couple of advantages. If its against a proper 8500, the 9000 Pro is gonna be pushed out that little bit more, but is an acceptable card in itself and only drops a small amount of performance compared to the 8500.
Although to be honest, I expect the 8500/9100 to age better compared to the 9000 cards due to the difference in pixel pipeline architecture... but who knows.