Of the two, I liked Pandemic the best. It gives each player a role which changes the way the game can be played. We only played on the introductory level, but even then we seemed to get quite lucky in the way the cards played out (we were able to cure two diseases before having a single epidemic card). In that sense, Pox seemed to have a better chance of winning as it was more logic based (although luck still plays a large role). I do think that Pox might be more scale-able (working well no matter the level of players) than Pandemic.
Considering my dislike of disease narratives, I was surprised that I liked these games. I liked the fact that both games had us working together against an emotionless opponent. The mechanics behind the spread of the "diseases" in both games followed a logical, yet still fairly random pattern that would make each game quite replayable. I certainly wouldn't mind adding either or both to my board game collection.