If a buyer were to consider both, what are the pros and cons of either.

You got some of your facts right, but are a little out of date on others. I assume you wish to publish fact and your attempt makes some good points. Please visit the website address shown for the facts. This is the official web site of the MU-2 and deals with some of the myths that have been floundering about for years. In addition, no one uses the old pressure relief method anymore. Pressure is relieved at the cap now for the very reasons you mentioned. The sniffle valve, which was used in the early days to relieve pressure has another more important function to regulate tank over or under pressure in flight. Pressure relief was a secondary function when the old, one step lock, fuel cap was used. Virtually all of those are gone now. Current training dictates relief at the new style Gabb cap.