I agree with the M3 sound. Sounds....whooshy. I prefer Hondas or the NSX by comparison.
I do love the M3 though, great performance for an NA combined with the usual BMW luxury with plenty of space. Not sure if you can get a convertible here in Australia but there are quite a few in UK (and going at good prices too). Personally if I were to spend that much I would go for the AMGs from Mercedes specifically the E55. 1/4 mile in under 13 seconds that thing hauls.
To add, I am very impressed with the Skyline 370GT, its a gorgeous looking car :
http://www.youtube.com/watch?v=_49C4S3hdlI