*nods*I would have said a Mk1 996 GT3 but that boat has probably now sailed (high 30's and up now).
When people realise that it doesn't have the same engine issues as anormal 996 and will leave a 964rs behind on a track, they will have negative depreciation for a while.
It's easy to forget what an exceptional machine the 996 GT3 is. But it takes less than a mile behind the wheel to remember and to fall in love again. It's also very rare - about 108 in total, 20 odd Club Sports.
You'll have to find £10k more than a decent 996TT but they're calcium carbonate and coagulated milk to drive. The turbo is a relaxing daily cruiser that goes a million miles an hour without even trying, the '3 is the last proper, finger-tippy, thrilling driver's Porsche for the road. I'd much rather have a Turbo for a cross-continent motorway blast, but when I got to the mountain pass the other end I'd be in the wrong car.
Do have a go in an 964 RS (or your pal's RS-suspended Carrera) before you commit down that route. They are just as hard as the press said on the road. If you're pressing on they can easily skip over mid-corner bumps and light up both rears. Not a bad thing when you're really on it, but they do demand considerable concentration to drive quickly so are quite a tiring daily motor.
964 wise, unless I was looking at it for 'investment' I'd get a tidy Carrera 2, fit H&R Green springs, RS cams, single mass flywheel, noisy exhaust, cup wheels and take some weight out. You'll get 90% of the driving experience of the RS but I'd say better cross-country pace through the confidence a more compliant ride gives.
A 993 will typically have lots of smallish (say £400 - £800 pa) bills for general maintenance followed by the occasional big one - like £4k for new suspension. The 996TT will have regular bills somewhere inbetween. It'd have to be a really bad 993 to cost more to run than a 996TT. As has been said, you'll probably get mid- to late-20s mpg in both and you'll insure a 993 for far less.
Truth is - only you know what you want, have a go in as many as you can.
