On the 370 side for your budget you could almost certainly pick up a pre reg one with Premium Connect, GT spec, a base one is 27k or thereabouts, without the GT pack and smaller wheels etc brand new.
GT pack new with a small bit of finance and you also have a bullet proof 3 year warranty and free servicing for three years at the moment.
M3 V8 is a stonking good car, but a four year old one? hmm, big money, I'd be constantly worried if anything went bang. Again a Cayman, if you choose that will be older, no warranty and the old shape, but a good car nonetheless.
Audi S5? There are that many A5's on the road that a S or even the RS get lost in their commonality.
The biggest downside to a 370, as you will be aware, driving a 350, is only two seats, running costs are relatively cheap and for a 3.7l car, mpg is very, very good.
Big shout buddy, but try them all then decide.