I have a original PS3 and mine was fortunate enough to be made after the "Yellow Light Of Doom" issue was resolved, so mines been going strong for 2 years. When the first slim lines came out and originals they had that issue..but personally( and I worked at GameStop when this system was the hot tamale so you can trust me here) now, either one works just fine if it's been made recently. If you go to buy a refurbished one however, ask for the date it was manufactured and if its still under warranty..just in case it ylod's on you, so you wont have to worry about paying out the rear-end to get it fixed at GameStop.
While there is nothing wrong with the ps3 slim (I have one and it works just fine), it lacks the backwards compatibility. If you can find one of the older ones they have either hardware or software emulation t o play ps2 and ps1 games. Wikipedia has a nice chart about 1/3 down the page of the different models and their features.
In my experience with the PS3, we got a slim and it stopped working about three months after we got it. Our friend sold us his PS3 which is an original for cheap. He has had it for a year or so and now we have it and it works great! For us the original is better.
I have an original 60 GB from the launch and it's been going strong ever since, but i do have an 80 from the recent production (not slimmed though) that works just as well but like it was said above, lacks the backwards compatability.
Also, pay the extra bit of money and get it insured with GameStop, so just incase the warranty's out and it YLOD's on ya..it won't be too bad expensive to fix at the GameStop.