I've planted alfalfa with both a 5100 drill and a Brillion Sure-Stand seeder.

Alfalfa needs a very shallow seeding depth, and how well and how consistently you get that depth will be the determining factor in how uniform a stand you get. The seeding rate for alfalfa should be 7 to 15 lbs per acre, which will mean your drill would be closed down pretty tightly. There's no way I could see seeding with a 510/5100/etc drill set for wheat, unless the alfalfa seed was coated with clay. I don't recommend buying coated seed. It's expensive and I never saw the advantage.

For me, the Brillion was a implement that needed a very well prepared seedbed. The top inch needs to be dry, but not too dry, or otherwise in more clay-type soils, the Brillion packers will load up with clay and it will just pick up your seed like a adhesive lint brush. With a Brillion, the field needs to be very level, or otherwise you end up with the Brillion bridging the low spots, leaving "puddles" where the seed isn't in good contact with the soil.

With the 5100, you need to turn down the rate, set the gearings, etc. In my soil in Nevada, I dragged a chain behind the drill to get some dirt over the seed in the shallow trenches the openers left behind. That worked in that sort of soil. YMMV.

The best drill I found for drilling in alfalfa was a Great Plains no-till drill. It resulted in by far the most consistent and uniform stand, with 6" opener spacing and individual closing wheels, it had the highest emergence rate of the 5100, Brillion, and air seeding.