Playstation3 or PS3 Slim?

I'm planning on getting a playstation and I'm trying to figure out what model is best for me. I heard the PS3 slim isn't backwards compatible, will I regret buying this model if I ever want to play ps/ ps2 games? Also if I got a PS3 which would be the best gb to get in terms of having a little bit of everything? (Enough room to play games, enough usb ports, etc, etc) ~ Thanks in advance