I think having a battery instead of a second alternator might depend upon what are your mission requirements. If you happen to be outbound on a long trip, like a vacation, and the primary alternator fails, you are going to have to get it fixed. The standby alternator, especially the higher capacity one, will actually allow the complete completion of the trip and allow for the repairs while back at home base.
We could aslo argue the other side of the coin--- what if the battery fails? Clearly, the second alternator probably isn't going to do much good. However, complete battery failures, while not unheard of, seem to be less than alternator failures. Battery technology seems pretty robust these days, as does the B&C alternator product line.
I hate to jinx it by making this statement, but the only alternator failure I've ever had was on my original RV-4 with a GM alternator. Been using B&C ever since without any problems. That's over 4000 flight hours across 5 different RV's all wired the same way.
Vic