IMHO Forster's Benchrest Seater die simply is a better mousetrap. The inside of the die replicates your chamber and is spring loaded. I am confident that it does a better job of lining up the bullet with the case mouth and ensuring concentricity.
Redding's Competition Seater is very similar. I've read that Redding actually uses the expired Forster patent, but there seems to be a slight difference in how the dies are made. The Redding costs a lot more.
The RCBS X-Die sizer also is a better mousetrap. With .308 Win in an M1A, I used to have to trim almost every case every time. With the X-Die, I trimmed once and have 2-3 more reloads per case before they have to be pitched. Not as big a deal in an AR-15, but working the brass less and making it last longer isn't bad.
All that said, you'll probably be fine with any 2-die full length sizing set from Forster, Hornady, Lyman, RCBS or Redding. (I don't have direct experience with Lee dies.) You didn't say if you were shooting competitively, going for accuracy from the bench, or just having fun. If just fun, buy the Hornady set and enjoy the savings.