Mr. Weiss,
I suppose, for me, it would depend on the campaign. If it was going to be a long running client or if it was going to be my company's website I would pick one phrase and run with it for a few months and then change it up and run the other for a few months and compare the data to determine which one was the most successful.
If, however, it was a short running client like a 'one time' optimization (which I hate but they do happen) I would first rule out that I could in no way run them both together and if I couldn't and if I determined that they both carried equal relevancy to the page's subject I would take the one or two words that make the short tail phrases different (in this case 'background' and 'credentials') and I would find out which of those carried the most search volume and make an educated guess from there - I know you touched on that and I agree that it doesn't 'mean' much - but it does help with getting an idea, in my opinion.
You pose an interesting question and as I have taken your advice many times - and you certainly give excellent advice - I am quite curious as to how YOU or others here would approach this. I will be watching this thread closely.
Cheers, my friend