Many people think that the Oly upper is more accurate than the BM because of it's tighter chamber and more uniform bore (no chrome lining).
The BM would definately be a better rifle for the field, since the bore would not be as prone to rusting. The military uses a chrome bore because of the potential for their guns to see hard use in the field.
What do you want to use this rifle for? If you are trying to build a tack-driver for varmint use, I'd go with the Oly upper. If you are just building a regular AR (which are plenty accurate anyway), I'd go with the BM upper.
HTH,
Gino