I'm befuddled by the text. It seems like maybe it's a joke but, if it is one, it seems kind of lame. "marginally less interesting" means "a little bit less interesting" but, if that's all there is, the exchange is rather boring and there seems little point in saying "marginally less interesting" than just saying "I'm not as interested in that". I'm wondering if this is meant to be a finance-related joke. "interest" after all has a financial meaning and so does "margin". However, as I said, if it's meant to be humorous, I don't get the joke.

To me, this sounds like something you would hear on Sherlock. It's definitely a joke, because the storyteller wants the person to ask where they bought the sandwich, but they don't care.

It means that the speaker has more interest in the reason for the bank visit than where the sandwich was bought.