Ok, so I've been looking a little more into this. My choices overall are 360, Gallardo, R8 or a magnum Cobra replica. The only "day to day" cars are the Gallardo and R8 so I would sell the M - if it were either of the other two, I'd keep the M.
360's do need belts servicing every 3000 miles or so and they are an expensive pig to fix if something goes wrong (I'm leaning toward a manual shifting coupe for the reason there is less that could potentially go wrong). Gallardos by comparison have chains (no belt service) and only need a service every 6000 miles. The R8 seems like basically a cheaper Gallardo but service wise probably cheaper - whilst I know its a fantastic car, I can't get away from the fact that the interior reminds me too much of "company car". Also, the Gallardo is slightly more current than the 360 but they are more expensive for a similarly good example as they're newer.
One thing that is starting to bug me about the M is the plastickyness of the interior. I have considered getting alcantara trimmed etc for for the cost of £2k that you don't make back on resale, I figure I'll let someone else take the depriciation and buy a car that already has this and/or other upgrades. While the M is a great car, I do feel that in a way, BMW just took the M3 engine and put it in a Z4 without that much consideration about how they could make it feel that much more special and for a £40k list price car, the interior is a little plasticky in a way that a creme 360 interior isn't.
This kinda decision does have to be a heart over head thing in some ways, but this was all answered for me when I sawa 360 over the weekend. They always make my heart stop in a way a Gallardo or R8 never does. They have that rarity edge that makes them seem all the more special in a way 911s never have for me. They are slightly older in terms of styling, but they're still stunning!! My only reservation is that its 2.5x the price and the power is almost the same as the M and that it may also bring bad attention in a way that a more subtle car such as the M doesn't... so its really a choice between that and a Cobra... Has anyone driven the 360? How did you find it compared to the M performance wise?