This is a tough call--I've played both on the PC platform, though not every single expansion. I think the Sims 2 is a better game, but it does take some more work (Sims age and die, so everything becomes a little more stressful) and taxes your system more. The graphics are much better and there are more features. I'm thinking about getting the Sims 2 for my MacBook soon, but am a little worried about the system requirements--I will not be buying the Sims 1, as awesome as that game was. I'd rather try to master the stressful parts of the Sims 2, rather than sacrifice the features/stuff in that game.
I'm just one person, though, and not a Sims diehard--any other thoughts?