Views: Show either field a or field b
put the logic into a views fields template


If it's a "one-off", most of the time, that's the way I would have gone about it. Display the "primary" field, exclude the seconday. Template the primary to display the second as necessary.

At a recent project, I "was just going to do this once", but then the client started changing their mind, and now I have a really big mess of views templates. If I knew that I would have a lot of logic like this, I would use a Views Context display instead, pull that into Panels, and have the Page manager handle the display logic instead. More complicated, but also neater IMO, and easier to maintain, as the logic becomes independent of the presentation.