I was going to go DA right off the bat, but as I've been leveling I've noticed that Zombie always give out more xp than any other mobs. For example: A green con zombie in the Ruins of Despair gives the same xp as a high lvl white con mob in Abandoned Camp. This isn't the only example I can give either. Because of this discovery, I was wandering if I am better off just going Paladin. That way I can level faster. I guess what it comes down to, is what the DA's and the Paladin's role is in PvP and which one I would prefer to do.
As of C4, neither class is particularly adept at handling 1vs1 or group PvP. They simply do not deal enough damage. The DA has a panther for assistance, but this panther can be dispatched as easily as a 2x HP mob (and its entirely too expensive to summon). Both classes need a boost, but generally speaking, the DA is slightly more popular and desired in PvP and PvE situations than a Paladin.
In C5 (which will be coming within the next 6 months, I assume), classes will be more equal for PvP (we hope).
Regardless, your process for looking at xp was logical but not particularly scientific ^^. In truth, the reason those undead mobs gave you more xp is because mobs that have additional abilities and resistances (such as the ability to cast spells, heal themselves, stun, buff) offer slightly more xp than mobs that don't.
AC mobs are limited in that they have few abilities and resistances-- only a few of them have self-buffs. The reason the AC is such a popular hunting spot is not so much the xp per mob but the quests that come with it.
In general, undead mobs do not offer higher xp than non-undead mobs. Also, the number of good hunting spots that contain undead mobs you can find at higher levels tend to dwindle. That is not to say that there are less undead mobs, just that they simply aren't effective hunting spots.
Also, keep in mind as either a DA or a Pally, you will probably not be soloing past level 40.