What is the difference between PS3 slim and PS3 fat version?

Im going to soon make a purchase and the guy is selling the fat ps3 60gb for 350 dollars (converted into dollars from Bosnian KM) and the PS3 slim 120gb for 550 dollars (again converted from KM to dollars). Now what is the difference between the two systems, if there are any at all, and i know the GB difference. so yea... Is the SLim less likely to overheat or some thing along those lines...