Do they both basically do the same thing? If so is one better than the other?
While the people that sell Milorganite will tell you that it will add organic matter to your soil you will never be able to add enough of it to make much difference while with the Alfalfa you may be able to. Which to use depends on how much organic matter is in your soil now and whether yoru long term plans are to keep that level or increase it.
I think of Milogranite as a less toxic substitute for chemical fertilizer. I use some to supplement the grains I put down. The grains last longer but cost more. My back yard where the kids play is fertilized solely with grains. It is thicker, greener, the soil is better and needs less water to stay that way despite the constant abuse it takes from the kids and dogs. Both grains and Milogranite take a while to kick in. Don't panic if you don't see immediate results.