If money is no object, the Granberg chain is supposed to be the best.
Most of us use the Bailey's (Carlton) ripping chain because it is a good value.
You don't need 50 feet of chain unless you plan on milling something that has embedded rocks and nails.
When I started milling, I bought 5 loops, thinking I'd be going through lots of chain. As it turns out, I typically use two chains in a full day of milling. Sometimes 3 chains. I haven't worn out a milling chain yet, either.
3 loops is plenty. I swap them in the field, then sharpen them on a grinder at home.
For a 120 cc saw, most people use 3/8 x 0.063". One AS member prefers .325" x 0.063" for bars up to 42", claiming it is a little faster than 3/8". The .325 milling chain and bars are a little hard to come by, though.