I'll be in the same position come July matey...
I've considered playing it safe, saving a bit of money and opting for a 335i coupe... Some dirt cheap ones about and it's a phenomenal car. Maybe too plain for some though.
I'm pondering the same couple of cars as you've mentioned... 997 carrera S and E90 M3 coupe. Totally depends what you need practicality wise i suppose, only you can decide that, and don't forget running costs. It's all good and well throwing around the idea of a Gallardo but good luck with servicing it and maintaining it out of warranty!! Not for me thank you!
My uncle has a 997 C4S and it's phenomenal (just come out of warranty so i might get him to sell me that). Very easy to drive and controllable about town so not bad for even commuting providing you can afford the fuel bills. Engine sound is just beautiful and lets be honest.... It's a Porsche. Servicing costs aren't insane compared to other 'supercar' marques either.
What is nice about Porsche's is that everything feels 'special' and low production...Unlike in a Lambo for example where you've got an RNS-E straight out of an A3 or an Aston where you've got switches from a Volvo or Mondeo.
The M3 is probably the best all round car in the world (IMHO). It just does everything perfectly and it's as quick as the Carrera.
If it was my money i'd have a 997 CS, if i needed a little more practicality....The E90 M3. Can't go far wrong with either really can you!
P.S. I agree with the Aston comments. Since the big corporate take over they just feel pretty uninspiring to me. I had a spin in an AMV8 last year and it definitely lacks the quality of a Carrera or Audi R8.