Allow me to add my 2 cents worth since i own and drive both every other day. U do not wake up one day asking yourself whether u should buy the M3 or the M5. They serve very different needs. M5 is primarily targeted at a higher age group requiring more space and comfort, but importantly, a more mature image. Of course, being an M car, u bet it will drive and go like any M car should. Ask yourself if u need more space than what the E90 M3 dishes out. If u don't, owning the M5 becomes something u do for the fun of it. For me, the E90 M3 would have served my purpose pretty well, but owning the M5 is on my bucket list, so here i am.
Another factor to consider is the power delivery. If u are sick of the high revving nature of the S65, the M5 is a welcome departure. I have to say i enjoy both
If u want the massive torque characteristic in an M car, but don't need the space and comfort, wait for the next M3. The F10 M5 never feels its size unless u are maneuvering in tight spaces. Despite what the BMW marketing material would have u believed, the M5 is not a track machine first even though it goes pretty fast around one. It would feel its weight on the track and consequently, feels less rewarding. U are certainly welcome to take it to the track, but don't buy one for this purpose.
As for fuel consumption, if driven like M cars should be, they both suck...