well...
There are differences in the "archival" qualities of each.... for example, I believe that all foam core is buffered, while there are lots of matt boards that dont need buffering (I'm sure someone will step up and explain the rationale for buffering..)

BUT - there is also the different physical properties of each... foam core is more fagile, but lighter, also more likely to be gas permeable.. and I'm not sure which would wick water faster, or retain it longer, or stain/warp more given equal doses of humidity/liquid...

Personaly, I prefer to mount on 4ply matt, and overmatt with the same thing. I think that for the cost, I feel a whole lot better about showing my work, it's just knowing that I didn't cut a corner, and that no one else will think I cut a corner.. and possibly raise questions about the sincerity of care for quality. Even though I often pinch pennies for lots of other things. ;.)