There is no evidence that serif or sans-serif significantly impacts readability. Alex Poole conducted a study on Which Are More Legible: Serif or Sans Serif Typefaces?. His conclusion:
What initially seemed a neat dichotomous question of serif versus sans serif has resulted in a body of research consisting of weak claims and counter-claims, and study after study with findings of “no difference”. Is it the case that more than one hundred years of research has been marred by repeated methodological flaws, or are serifs simply a typographical “red herring”?
It is of course possible that serifs or the lack of them have an effect on legibility, but it is very likely that they are so peripheral to the reading process that this effect is not even worth measuring ( Lund, 1999 ).
Indeed, a greater difference in legibility can easily be found within members of the same type family than between a serif and a sans serif typeface. ( Tinker, 1963 , Zachrisson, 1965 ). There are also other factors such as x-height, counter size, letter spacing and stroke width which are more significant for legibility than the presence or absence of serifs. Poulton, 1972 ; Reynolds, 1979 )
Finally, we should accept that most reasonably designed typefaces in mainstream use will be equally legible, and that it makes much more sense to argue in favour of serif or sans serif typefaces on aesthetic grounds than on the question of legibility. ( Bernard, 2001 ; Tinker, 1963 )
There is no evidence that one type of font should be used for either - serifs do not provide any guidance to the eye. From Alex Poole's study, again:
Serifs are used to guide the horizontal “flow” of the eyes; The lack of serifs is said to contribute to a vertical stress in sans serifs, which is supposed to compete with the horizontal flow of reading ( De Lange et al., 1993 )
These are the most common claims when trying to make a case for the utility of serifs. However, serifs cannot in any way be said to “guide the eye”. In 1878 Professor Emile Javal of the University of Paris established that the eyes did not move along a line of text in one smooth sweep but in a series of quick jerks which he called saccadic movements ( Spencer, 1968, p. 13 ; Rayner & Pollatsek, 1989, pp. 113-123 ). Unfortunately many graphic designers and typographers continue to use this rationale for the existence of serifs, due to a lack of communication and cooperation with the research community.
What you should be looking for is a font, serif or not, that has been designed for legibility. If you are dealing with text on the computer, finding a font that is focused towards computer display legibility would be preferred. For example Microsofts Segoe UI font (I use this simply as an example, not a specific suggestion - especially if we are talking web).