Wizard, no doubt. Witch Doctor is fun- even has a lot of the same function as D2 Necro, but the feel and use is slightly different than a Elemental caster/powerhouse. Wizard plays right that way and works almost exactly as you would expect a Blizzard designed spellcaster to play. Right down to Frost/Arcane/Fire trinity.
I love the wizard, it's going to be my first class to 60 and the class I preferred by far the most. Wizard in the beta is very pew pew pew, with some freezing abilities, some aoe abilities, but emphasis is mostly on kill it quick.
The witch doctor has a lot more spell graphics and a lot more different spells, it felt totally different to me - I relied heavily on my dogs and I found the whole throwing jars of spiders / summoning frogs kinda weird myself. Fun, for sure... but I prefer a more direct style of damage. I think if you're a necro fan though you will enjoy WD a lot.
If you want to play something similar to your mage in WoW, go for the wizard 100%. If you want to play something similar to the necro in D2, definitely the WD.
Witch Doc is a very strong class currently. As always that will change from time to time. We also dont really know how the other versions will effect classes. Once we get into hell modes there is certainly going to be classes that do much better on their own and others that are rockstars with a group set up. The best thing about all of the wondering and debating is very soon we will know for sure. Man the middle of May cannot get here soon enough.
Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before... He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way. -Kurt Vonnegut, "Cat's Cradle"