For PvP, Alliance has more arena players. For PvE, horde is a bit better. But it's not like horde doesn't have PvP players or Allies don't PvE. Good guilds of both on both sides as well. The balance is slowly shifting, as the cycle usually goes. Total number of players on each faction are similar. You can check the Information page (on top) to check the actual figures.
For leveling, the XP rate is x7 so you will reach 90 pretty quickly compared to retail. Every zone has enough working quests to level out of its range. Almost all zones have much more. There is no instant level 90 vendor.