I would say the M5, I actually considered two of the cars you mentioned, M5 and XKR, thing is though you then go on to say you don't want an expensive car to run which all of them are, I have been looking at M3's and M5's and a guy said he had just spent £800 on front brakes in the advert, albeit the bulk of that was probably labor, even DIY as I would, I suspect the actual parts are still expensive!

I do like the both variants of FRS as well, XK is good but I don't like automatic cars, no true sports car is!

It seems E39 M5 is cheaper than M3's from what I can see?

Plus I think the M5 is more powerful from memory, I think in fact XKR is around 370HP and the XK8 around 300HP.

Martin