I have just gone for a 997 C4S, I could have gone the 996 Turbo route but a test drive in the 997 convinced me of the progress Porsche have made across the board in terms of the driving experience. The 997 interior is also dramatically improved over my last 911 (a 964) and is indeed a move on from the 996 as well.
There are different types of car owner in the performance leagues. I think Porsche are markedly different to the high bhp Japanese cars and actually there is a distinct difference in the migration paths. A lot of Japanese/turbo/chipped fans move between 350Z, Evo, Impreza and maybe end up with a Skyline. A lot of 911 owners come from the M3, RS4/S4 background it seems to me. I have had a couple of Japanese cars, a Supra and a P1 Impreza, both were fantastic but I returned to the German marques because I think the focus on pure bhp is not as important as the driving experience. Straight line speed in the UK is not easily used anyway and in any case, the C4S is hardly a slouch when you press on. I felt the greatest sense of acceleration in my P1 Impreza, more so than the 911 and more so than my previous M3s. The C4S does give a real sense of the car accelerating and this is important. I once had a painful few months in a Jag XJR - it had huge power, superchargers and plenty of performance - felt like a hearse though...
I have a Caterham 7 and it will easily outperform a lot of Japanese machinery and 911 owners on a trackday. I drove Spa and found the 911s beat me in a straight line but held me up in the bends (new engine for the Caterham goes in soon !). The C4S brings really very good handling to the 911 and a test drive should leave you little doubt as to what you want.
If you love hooning down the slip roads or away from roundabouts with your foot planted, I think the 996T is superb. If you want to really have a great drive through the lanes, the C4S 997 might win out. X51 is your easy way to step up and I am looking to do this soon to my C4S.
Make sure you get the PSE as most Jap cars sound great compared to the stock exhaust on a 911.