The Slim PS3 is much more reliable than the Fat PS3 and they have ironed out lots of issues primarily the YLOD and Blu Ray laser failures. The Slims however have less functionality (no card readers, SACD Playback, PS2 playback etc..). I would say get a slim one if you haven't bought a PS3 yet (dont get a used fat).
However seeing as you already have a Fat PS3 why don't you just use it till it dies first. There are many ways to keep it going as nick09 touched upon. Just stick some Arctic Sliver 5 on the chips and that will pretty much protect against YLOD. I've been using my machine very carefully since the laser went.
It sucks to have to worry about these things when in previous generations of console I never batted an eyelid as to how they worked, what was in them and if they would fail if used too much.
Robin.