As others have said, it really depends on if:
1) You want to play PS1/PS2 games on it
2) You want to store media/downloaded games on it.

I've had mine since 2007 and I have only filled up ~40GB. I have also read that it is relatively easy to upgrade a PS3 hard drive, though I can't tell you if that is computer modding person easy or real person easy.

Also note that the slims run cooler and quieter, if that is important to you.