I have both the Predator and the R-15 ( .223 with a 22" barrel, non-collapsable stock). While they both are "manufactured" by Bushmaster, there are some differences.
The Predetor has a 20" barrel vs. 22" for my model. The predator barrel is certainly thicker than the R-15's. The predator is slightly heavier too. The predator will shoot 5.56 and .223 while R-15 is stamped .223. (Some will argue for both sides of what will shoot what so I'll drop it) The R-15 has a 1/9 twist while the predator is 1/8. The both shoot 55 grain V-Max superbly, but the 1/8 Predator will shoot 75grain Hornady BTHP Match like it's nobody's business.
The fore-end are slightly different with the number of cooling slots and the R-15 is tapped for rails/acces.
The bolt carrier in another area of difference. The R-15's key is staked differently than the Bushmaster. The Bushmaster is chrome-lined while the R-15 is standard. While this may not seem like a big deal, and I've never had a reliability issue with either, the Bushmaster is a heck of a lot easier to clean when taken apart. (I'm anal about cleaning my guns so this might really be a non-issue)
I bought the R-15 in August of '08 and the Bushy in January of '09. I've never had any issues with either. They are both excellent calling guns and ergo-perfect. On the bench, I wish the R-15 had a better trigger, but in the field, where it counts, it works. I wish the Predator was camo'd, but there's ways to fix that. I paid $999 for the R-15 at Cabela's and $1200 for the Predator at Gander Mountain.