The 3" magnum ammo costs more and kicks harder because it has a larger shot charge and sometimes even more powder as well. People who are hunting waterfowl or turkeys can perhaps justify the expense and added pain, most other folks except macho masochists and people who insist more/bigger is better get along fine with standard 2 3/4" loads for general shotgunning. There are 2 3/4" maggie numb shells as well, see the remarks about 3" maggie numbs above.
You pays your money, you can buy whatever ammo you want tho... nevermind old fuddyduddies like sm and me, or that Dave McC guy if he shows up. Steve and Dave have likely spilled more shot than I have fired, I especially wouldn't listen to them of all people... 8^).
lpl/nc (who really likes reduced recoil 2 3/4" buck and slugs)