4100 - load both to 63-65k psi and get back to me on all bullet weights. When you hit 3250 with 140's and 3050 with 160's in your 280, call me. "Efficiency" diminishes as you burn more powder - in all cases. There is no inherent efficiency in the 280 case or any other case - the bigger the case the greater the point of diminishing returns when adding powder.
I've loaded for both for 25+ years. There is a distinct ballistic difference between them. Might agree that in the field difference is more limited but I've not shot enough truly big game with both to offer a valid empirical opinion.
