Community > anything else. With a higher population, there's more activities going on and more different types of players. You still have to find the right sub community (FC/LS) for you. The site you linked is a rough estimate. What you are going to notice more is if that particular server is active at the times you tend to play. I was on Diablos for a bit. It didn't feel as active as Leviathan to me but it could also be due to the time I was on vs actual server population.

I love my server, it is active and lots of people are on when I am on. I also have a strong social FC and LS community that I belong to which keeps me doing stuff with people I like to game with. If you are on Levi at some point you can shoot me a tell. I help run a more social FC with plenty of MMO newbies and vets alike.