I ended up from a 543 HP highly modified 996 Turbo with a 997 Carrera S with 20 mm chassis. Did I regret it? Definetely YES. Do I love my 997 Carrera S? Definetely YES.
Would I go back to a 996 Turbo after owning the 997 Carrera S? Probably not because there a lots of things I like on the 997 much better like the much nicer interior, the exterior of course too, the second generation PCCB (which is a treat in my opinion), the PSE, the chrono sport package, the new PSM setup, etc. What I miss from my 996 Turbo is that BRUTAL power I had all the time available at my throttle foot. This is why I'm trying to get a 997 Turbo sooner or later when it'll be available.
Yes, both cars provide a lot of fun but you really have to sort out what you want. Do you want brutal power? Go for the 996 Turbo. Do you want the newest 911 generation with some substantial steering/suspension improvements, not to speak about exterior/interior changes, go for the 997.
For most people, the 997 Carrera S power output is just right, especially for those who come from a BMW M3 or a Mercedes SLK/CLK AMG. The 996 Turbo is a different league, it can be a very dangerous car too because I assure you that you're always faster than you think you are. This never happened to me in my 997 Carrera S.
I know it is a tough choice, I had to choose between the 997 Carrera S and a 996 GT3 last year, both at the same price tag. I went for the 997 Carrera S instead and the 996 GT3 still sits in my dealer's showroom...for more than 10000(!) Euro below MSRP.
Another possibility would be to wait for the 997 Turbo and save some more money until it shows up.
Good luck with your choice, you can't really go wrong.
BTW: you should also take in consideration maintenance and insurance cost, I think they both are higher on the 996 Turbo. A 996 Turbo engine costs half a fortune if it has to be exchanged, 997 parts are much cheaper to replace.
I would NEVER buy a 996 Turbo from a private person because you can end up with a lot of repair cost. Buying from an official Porsche dealer may cost you more but over here in Germany you get a used car warranty which is worth gold. I suppose it is the same in the US.