Aberdeenshire hands down for me because of my enchantment with recumbent stone circles. A week is good time to cruise round a few different clusters, especially with a car. Public transport / cycling would require more planning. There's also lots of other ancient stuff. Tomnaverie / Sunhoney / Midmar circles is an excellent day out, with add-ons available such as Culsh souterrain.

Kilmartin Glen is also incredible! Make sure to check when the museum is open. For rock art that would be your first choice. I "did" the main sites over a weekend including walking round the glen in the full moon which was lovely, but a week there would be well spent since there are plenty of other beautiful places nearby such as the Tayvallich peninsula.

Either way you made the right choice!