Nobody on this forum has driven the E60M5 to give you any personal thought. I think the biggest factor is whether you want stick shift vs SMG III transmission. I predict that the people (me included) who had put down a deposit and eventually end up NOT buying the E60M5 is mainly because it does not have stick shift. Performance wise the E60M5 will become the king of sport sedan, there is no question about that. The price should be very similar to the E55 AMG so there should not be a surprise there. The 996TT is a perfect sport car, but with the introduction of 997 and then the 997TT in a year or two, its resale value may be affected. This should be considered depending on how long you want to keep it. Happy Motoring!
N.B. I have driven both the stick shift and tiptronic 996TT and find the car very fast and balanced obviously, way faster than my 993. But yet feeling a bit frustrated in city driving as you have to slow down right away after shifting into second gear. But I guess this applies to all the other fast cars out there.