Like Many Whelps said above, it could come down to what you want to do. If you plan to run your own 6-Box group, definitely consider the SK or Pally as your group's tank. If you want to raid with other human players, there might be an over population on any given server. An alternate idea is to make a shadow knight and create a 6-toon team around the shadow knight. In that team include a second class you might want to group with human players in a raid type environment. It might be the best of both worlds. The only downside would be if you had your heart set on raiding with the SK and there was no demand for raiding SK's on your server.
When the day is done, I think most if not all the folks here at Red Guides will say, have fun and do what will bring enjoyment to the game for you.
Be well. Happy gaming.