When i started reading the thread i wasn't understanding the OP. I figured it perhaps just over my head, but the terminology seemed a bit funny. Then i got a better understanding of what he was asking and promptly agreed with neuroanatomist and his side. This might be a hard thing for some to get their head around.
Yes Kudos to Neuro for answering an otherwise vague question.
I don't think it is a hard thing for someone to get their "head" around, as much as it is a scenario that would very seldom come in to play. In very few instances would the comparison of two lenses come up like this. I have heard this discussion on a forum at least one other time. But since you have a different FOV for your background you will never be comparing identical pictures. Usually a person thinks about proper framing first.
I suppose if you were big time and had a shoot coming up, knew the location you were going to and what type of bokeh you were shooting for it would help in determining which lenses to take along. Personally I don't have that problem, all my lenses fit in a few bags I just throw them all in and go. Neuro might have a little more difficulty and have to think about how much of his kit to take along.