I've been told by someone who worked with a researcher that Sans Serif fonts have lots of ambiguity in things like i, 1, l and this causes the eye to need to back and forth (more, presumably than the usual saccades).
The evaluation criteria is: * Reading fatigue (how much can you read without fatiguing) * Retention
Now, one counter point to this, years ago, might have been that low resolution screens may not render the serifs very well. (And this person indicated that even back then serif fonts were better).
So, I"m wondering, what the latest research is showing, especially for those older folks with poorer eyesight.