Actually, the '03+ S has different (bigger) sway bars than the '03+ Base. Minor detail, I know.
Another thing is that the S also comes with a lot of interior bits and pieces standard vs. on the Base where they'd be optional.
I have an S and I still think it could use
a lot more power. About 100hp...but more importantly 0.4L more displacement (e.g. the 911's 3.6L motor). I test drove a Base model and the difference in torque vs. the S was very apparent, especially below 4.5K rpms. I don't think anybody will say they have enough power, no matter what car they own. Once you get used to the speed, you're ready for the "next step". Don't get me wrong, both Base and S are fun cars. But when you have cheap econoboxes with 300hp/260+ torque (WRX STi, Evo 8) doing sub 5 second 0-60, a 6 second Porsche seems...not right. I think we Boxster owners deal with this issue because everything else about the car is damn near perfect.
One important aspect that people seem to forget is that although the S costs more initially, the resale is also higher. So hypothetically speaking, it actually
could cost you more to own the Base model. Food for thought...