S for sure (if you can afford the extra Pounds). There's a fairly long list of items that are included on the S which are optional on the standard non S car. If you would choose those options anyway, then the S actually becomes a cheaper way to get those options.

e.g. Bixenons, 19 inch wheels/tyres, self-adjusting clutch, more powerful 'red' brakes, alu-look trim, aluminium dials, sports steering wheel, PASM/-20mm & LSD, helmholtz resonator etc etc. Check the 997 Product Information Guide (which can be downloaded elsewhere on this site for the full list). Plus the S has the possibility of the factory fitted/retro-fitted X51 powerkit.

But, honestly, if you'll hear my advice, please don't approach this decision from the approach of what do you get for how much money. No, it's more subtle yet obvious than that. The two cars are different because, at their heart, they have different engines. The non S has a M96 engine. The S has a M97 engine. They deliver their power and torque differently. The non S is more linear. The S is more thrilling IMHO. If you pick Sport Chrono (which I would, and did, pick), the S becomes more alive in its responses.

Sorry if I am droning on. IMO, S all the way.