For me, it's a tough draw. I bought the 80gb because of it's larger harddrive space, backwards compatibility, and other features. I do say that having more than two USB ports is a life saver considering I use not only a controller connected to it for charging, but also a USB headset, keyboard, and EyeToy camera, filling up all the ports rather quickly. I did not think I would need BC because I have a slimline PS2, but given that everything is in one nice package without needing another system hooked up to my television set, it works out, and my games look nicer on a HD set. Also, I can download gamesaves and get a little more replay value out of my older games, like stealth camo in Metal Gear Solid 1-3, which makes the game fun to play around with.
The whole thing about the 40gb using less watts per hour is great, but when people are hooking up both systems (PS2 and PS3), you kind of defeat that purpose when you have both plugged in (I think). The 40gb is a great system and one that I did considered for a while. When I pay so much for a system, why not go all out and get something that's functional and at the top of it's game. The 40gb system kind of is bait to lurer more people I think. I don't know. Anyways, that's my two cents.