@Ken Fraser - that "depending on the content" is the crux of the matter! Do particular types of content call out to you for one or the other? The article I was referring to argues that late medieval scribes working on vernacular manuscripts did not make these decisions randomly, even if they may have been unconscious/subconscious choices.
The really interesting observation is that in some manuscripts, we witness the same scribe using BOTH the secretary and the two-compartment "a" - which I think most contemporary calligraphers like you or me would tend to avoid.
I would be so fascinated to know what your thought process might be - or if you are even aware that you have a decision process (it almost sounds like you make subconscious decisions about them). Do you feel some types of blackletter go better with some subjects than others? Even if you can't quite put your finger on why?
--thanks again for your input! K