Having had an 81 in the same guitar I currently have a MM in, I agree with the above from MMM and TIm, but would say that the MM is certainly heavier sounding, bass accentuates more in palm mutes and chords sound richer and thicker.
The MM is much smoother and more fluid. Its less compressed sounding as well.
I'd estimate the 81 has 15-20% more output (subjectively, of course), but its all in high end sizzle, the MM is the thicker, heavier sounding pickup.
Edit: I need to read harder and sleep more! Apologies for the useless repetition of info in this post!
Something else to note about the MM/81 comparison: the MM has much better cleans!