Ps3's failure rate is very low so I'll go with the used one also, the old ps3 it's a little faster as Blu-ray and dvd player than the slim, I know this because my brother works for a tech magazine they have made some some tests, however it's not a great difference.On the other hand the slim consumes less Kw/h and +40GB hard drive can be useful if you like to download many demos and PSN games
Used Ps3 Pros:
-It's chaper
-It's a little faster as DVD/blu-ray player
New Ps3 slim Pros:
-Lower power consuption
-Larger Hard Drive