Re: 911 c4 vs. c4s
The C4S is worth it in my book for the added HP and torque. Gives the car more grunt from a standstill and above 4,000 RPM a noticable improvement in excelleration by helping to overcome the added weight in the drivetrain of the 4 wheel drive. And as Topspd states above, there are a number of standard features on the "S" that don't come on the non "S", the best of which are the PASM suspension and big red brakes in my opinion, that add value to the car and could not be upgraded after the fact at anywhere near the premium you'll pay for and "S" over an non "S".
As for options, Sport Chrono is a popular, if a little overrated, option. Heated seats area must if you live where it gets cold. Nav can be added later but you can get it for free if you can find a car that has it. Full leather in a coupe would work for me. In a cab it's not so weather friendly should you get caught in the rain with top down.
'07 C4S Silver/Black/Black - manual, Sport Chrono, Bose, Nav, Power Seats with crests