Stick with the M1.
I ran the M5 a few times, After a little tuning I picked up .03 and 1/2 a MPH but I spent most of that season set up for Super Comp so it didn't matter.
With the M5 I noticed a pretty severe reaction to un-coated/un-anodized aluminum. If allowed to sit in my aluminum cell for more than a couple of hours it would become hot to the point of rolling itself around in the tank.