Works like this: The M1 has the familiar .25MOA per click adjustments that we all know and love from many other scopes. It's great for target shooting, because you can dial your zero in to within 1/4MOA, which is far finer than most people (or rifles) can shoot anyway. It is slow, and requires that you either have your comeups memorized or written down, and it has the fatal flaw of not having enough adjustment to move from 100 to 1000 yards in only one turn of the elevation dial.
Enter the M3. A fellow at Leupold (whose name escapes me at the moment) analyzed the above set of problems and came up with the following solutions:
1) the elevation turret would adjust in 1 MOA increments, enabling you to adjust between 100-1000 yards in one rotation of the dial
2) the turret would be matched to a specific round at a specific muzzle velocity, so that "comeups" as we learned them would be unnecessary. On acquiring a target at 400 yards, you simply rotate the dial to '400', hold dead center (yes, I see you in the back waving your hand talking about wind. Shuddup for a minute, I'll get there.) and shoot. If you haven't fucked up your range estimation you will be rewarded with a center hit.
What does all this mean to you? Depends. If you really think of your rifle as a people-puncher you're better off with the M3. Its adjustments are less precise, but much faster. If you go to a sniper comp, you're going to have problems with the M1 - eventually you'll move from 400 to 800 only to find out you're one full rotation out from where you need to be, and miss miserably.
YMMV.
QS