Trading in for the Slim is stupid. You lose the ability to install Linux, and you get an HDD that is 40 GB bigger if you go to best buy and get your $100 trade in deal, meaning your paying $200 for an extra 40 GB's of space, and a (debatable) better looking case.
I'd take the $200 and buy a 64 GB SSD for my PS3 instead, or maybe a 500 GB 2.5" drive, or maybe a slew of new games.
To be honest I like the look of the old PS3's better