well i just tried out os9xgp today (just got a card reader), and i loaded up mario all stars + super mario world, and the speed seemed really decent (no sound though). i loaded up secret of mana at 16bit without that video correction thingy, and the speed was playable i guess, i mean the only games i'd play on the snes are rpg games, and only a few (chrono trigger, secret of mana, secret of evermore, maybe zelda, etc., and maybe harvest moon) i already beat mario rpg like 4 times so i dont wanna play that again (though it WOULD be really awesome if it worked flawlessly on the gp32)
as for genesis, there are a ton of great games, not too many rpgs though. also, gbc also has its selection of pretty decent rpgs i guess, the zelda series has always been good.
genesis is great, drmd is just awesome, but my gp32 only clocks to 166, so there _are_ some games (big ones, like nba live, virtua fighter, etc.) that sometimes run at sub-par framerates, but works great if i turn sound off.
overall, the gp32's a dream. personally though, i wouldn't get a BLU just for the fact that battery life is short, and it's alot more expensive. i'd rather go ahead and go on ebay for a few weeks or so, and bid on a FLU, i got mine for less than $130+shipping. so... whatever fits your thing