I used to have a fairly basic set of criteria (when I was crediting miles to Delta) that by my valuation of miles and points, maximized my overall return (not taking into account a preference beyond value/point or value/mile). It went like this...
All stays over $500 - Points + Variable Miles
Hilton/Doubletree/Embassy/HGI under $500 - Points + Fixed Miles
Hampton/Homewood under $500 - Points + Points
This was before I came across BMI's Points + Fixed Miles of 1000/night regardless of what property type you're at. Since then I've pretty much let it ride, except during Delta MQM promos.