Personally, if I had no PS3 right now, I wouldn't get a used one even if I was going to get better BC, because a used one could have been treated poorly, and it does have a higher chance of failure (especially when being almost out of the warranty or out of the warranty). I'm actually a bit worried about my 20GB right now because the number of PS3 consoles breaking (launch ones) has gone up quite a bit at a few PlayStation forums. At least with a 40 or 80, you get a decision revision with less components (less components means less chance of failure).
Overall, I just wouldn't take the risk and would get a 40 or 80 instead.