You won't really find a consistently high population on any of the PvP servers - more people would rather level in peace, and PvP when they want to, rather than be subject to the whims of gankers, corpse campers, and so on.
That being said, I live in Long Beach, CA, and I play on Lightning's Blade, an East Coast server. The ping rates are fine - I really don't think that they have facilities in each time zone.
Lightning's Blade seems to be Alliance-heavy. I say seems because perception can be misleading. But we Horde are constantly being attacked at our outposts, like Tarren Mill in Hillsbrad, and rarely mounting counteroffensives ourselves.
Whether you want this information to join the underdogs, or be part of the "winning team," doesn't really matter... make use of it as you see fit.