As already said, the game is not due to come out for a fair bit. In fact, the characters have not all been released yet. I could go on and on about how awesome a certain class is, only to have them bring out a far cooler class in the future.

On another note, the three classes currently viewable are Wizard, Witch Doctor and BARBARIAN.

Or at least a generic brute class similar.. It definitely isn't the Paladin, but I definitely know the move list is based at least slightly on the Barbarian class from Diablo 2

Classes have many differences, so you can't just pick one because you can!

By default (and should always be portrayed as such) wizards are weak in physical strength and defense, but are strong in (obviously) magic. Usually, wizards are the weakest/hardest at the start as they can not take many hits, and their powers take a while to develop. Usually their defense is increased through magical barriers and shields. Each game (brand) also changes how their magic points are replenished- through items you buy, by resting/eating, etc. This can take a lot of money at first.

Paladins are great because they are powerful at the start, though the prices of good weapons can be steep, as they should be. Good strength, stamina and dexterity.

Witch doctors, I'm guessing are similar to the sub-classes of mages/wizards/warlocks: like a druid, for example, or necromancer. Probably has the same basics.

I usually go for physical attacks/melee classes, like warriors or rangers. I find that mages/wizards and their magic to be too slow to replenish, I prefer weapons such as axes and bow & arrows over a skill that can easily be depleted and takes a long time to recover, as usually the SP/stamina for paladins/warriors/etc can be recovered in a short time via food, resting, sleeping, etc.

The game won't be out for months. Why choose now?