g0at, Depends on what you want to do.
Is backwards compatibility or running Linux important to you? The new slim models do not support either with native firmware and all slim models have hardware removed to play PS2 games.
A majority of users simply don't care about either feature - so for them, a slim PS3 is the best bet. Because there is less "stuff" inside, they run cooler and are generally less prone to problems caused by overheating (YLOD, problems w/blueray after firmware updates, etc. etc.).
If running Linux is someday important to you, it is entirely likely that eventually someone will defeat the firmware limitations to running Linux.
PS2 backwards compatibility is best left to hardware and the slims do not have this hardware inside (it has been removed) so it is unlikely that a firmware hack will cover all bases for backwards compatibility (BC).
In my opinion, if BC was important to you then save the premium for buying the older models and purchase a cheap 2nd hand PS2 along with the slim.
If official Linux support (or not willing to wait for a hacked fw update - may take a while) is important to you, then consider the older fat models.
Otherwise, (IMO) the slim is the most stable platform for running PS3 games.