The big difference is that the 911 is the flagship.  It gets better motors.  It's also a 2+2 coupe with the engine hanging out the back.  Some like that, but it has tricky handling.  The 911 has many variants, but mainly it's S (upgraded horsepower, bigger rear wheels), 4S (S with all-wheel drive), Turbo (yet more power and all wheel drive), GT3 (racing variant, two wheel drive), GT2 (racing variant, basically a really nasty turbo).   Then there's targas and convertibles of some of those. 

The Cayman is a two seater and the motor is mid-engined.  The chassis is much better, which is why a lot of people swap 911 engines into them.  There's no LSD stock on the Cayman either, because Porsche tried to make sure that it didn't set better lap times than the 911.  I didn't install an LSD, though the previous owner might have -- it always felt like it had one, but I didn't want to pay to verify. 

You also get a front and rear trunk on the Cayman that you don't get on the 911. 

BTW, this car has been around H2R a time or two.  Not a member, but I know the owners well and my girlfriend used to run a High Performance Driver's Education school there.  It is indeed, quite fun around that track.  It's a pretty raw car, though no Lotus Exige.  It does have power steering and is pretty damn useable as a daily.  Honestly, I love the car and I've spent a year debating about selling it because I don't know what I'd like more.  It's that good. 

I could meet you in San Marcos if this is a deal you want to do.