Personally, I wouldn't say the driving experience was that far removed. Performance is very similar. The RS4's steering is weightier, traction is better but the 997's more nimble (all as would be expected). Overall, they're both brilliant drivers cars, but I'd have to say I actually prefer the RS4 marginally.
Reliabilty and ownership cost wise, both have been about the same for me, with the Porsche giving better fuel economy (5 mpg better on average), but costing twice as much to insure. No major problems on either.
Obviously the RS4 is more practical, but the 997's pretty good - you can get a fair amount in the deep front boot, and golf clubs will go on the rear parcel shelf with the seat backs folded. No issues comfort wise.
The biggest disappointment you'll have with the Porsche is the sound, so if you can find one with the sports exhaust, all the better.
I like the Porsche's compact size, and it's quite a nice looking car. However, if I had to choose one or the other, I'd actually go for the Audi I think, but that may be because I've had several Porsches.
As said above, if you've never owned one before, take the plunge and own an "iconic" 911 at least once in your life! If I did get another one I'd probably go for a C4S next time, as I'm used to the RS4's 4wd, and the fatter arse looks sexier as well...
Make sure the 997 comes with a warranty, though. As said above, Porsche only give you 2 years from new, but if you get it from a delaer it'll have at least a full year on it.I think the biggest area to watch for is electrical gremlins, but they are pretty well built.
So find yourself a good one, and enjoy! Here's a quick pic of the one I had ...
