You don't want to be on a low pop or a high pop server, those two extremes will not be fun to play on.
Low pop = ghost towns with hardly any activity or players to group with.
High pop = Sometimes you will be forced into a waiting queue to join the server because it's full (usually at prime time). There will be so many players about that questing is difficult and on a PvP-high pop server you can expect to be ganked every ten seconds.
I'd go medium population, to get away from those two polar extremes.