You will have to do both over time, naturally. The thing about 'Kilmartin' is the excellent stuff in the immediate outlying locale, not just the glen itself. Could easily spend at least a week, whereas in Aberdeenshire at least a month to just see the main sites - just ask Drew.
If it was me forced to make a choice, I guess I'd go with Aberdeenshire first off... Sunhoney, Loanhead of Daviot, Tyrebagger, Aikey Brae, Easter Aquhorthies, Finzean, Cothiemuir Wood.... speak for themselves.