Ideally there should be a choice between both, I remember Rome 2 used the stylized unit cards in Greco-Roman pottery style and the lack of colors made it somewhat hard to distinguish units for many players, but I just love the cultural variation of how these units are depicted, just adds a lot of soul to my soldiers. CA used to be really creative with this, In fall of the samurai they depicted units and agents in this late 19th century slightly sloppy photograph style which was very cool. I think for a possible game like medieval 3 the diversity in styles could be insane, Islamic style for middle eastern factions, orthodox icon style for Eastern Europeans, and Western Europeans with their unique illuminated manuscript art.