It's usually more of a gut call for me; I don't calculate cpms. However, the respect in which I make a de facto cpm decision is that I almost never use miles for domestic award tickets (except for a thankfully rare emergency situation or occasionally getting a ticket for someone I'm close to), instead using them for international J and F tix (generally for vacations with my wife) because they have much better cpm value (and I would never be able to afford most such trips paying $$). I do sometimes use miles for domestic upgrades for trips greater than two hours, but that's because I have a lot of miles to spare and really don't like flying coach in terms of both comfort and getting work done.

Or to get back to the way you framed the choice, I definitely opt for relationships, though I'd put it not in terms of cost but in terms of VOR (value of relationships ).