manual 6spd vs. 7spd SMG:
fuel mileage - SMG. With 7 gears you have a larger overdrive ratio which brings the revs in top gear much lower. ( to the top gearheads, i'm sorry if this sounds obvious. this is meant for someone who is wondering why you might need 7 gears). I know that on the FW26 this gear drops the revs by 500. I really don't know if the ratio was changed for the M5 so just how big of a reduction this is, is anyone's guess. A drop in revs is a hike in MPG and engine reliability. There's alot more stuff to list but not enough room.
interaction- 6 spd. clutch, shift, gas. repeat until satisfied
long trips- SMG- either pull the paddles, slap the shift knob or leave it in automated mode and set the cruise. this is also an advantage in case some cuts you off and you have to panic brake, no worries about stalling out on the freeway. ( stalling out like that on the freeway sounds dumb, but depending on how bad you panic, your left leg may freeze, especially if it's on the foot rest)
racing- SMG- hands down. more consistent launches, faster shifts, and less chance of a missed shift.
stop and go traffic-SMG- computer controlled clutch = less slip. and a hill hold program helps in hilly cities
starting off- this is going to vary in the SMG depending on the shift program, but i think the 6spd is going to get this one.- At the present moment, there is too much lag in the SMG for my taste. And forget idling forward, if you aren't pressing on the gas, the car won't move.
these are just a few of things to think about. of course this is all IMO. everyone's individual tastes will vary.