I started out leaning toward Archai because Wiz uses a lot of Fire abilities, so attunement should be great. But the more I think about Ash Elf and Gnome, the more I see them as very close 2nd choices.
I am an alt-a-holic who wants to play every race and I have other classes I'd like to use both Archai and Elf for. And I don't plan to play any other class that Gnome is particularly good for. So unless the actives make me shy away from Gnome, I'll likely go with that.
All comes down to the values of these bonuses - humans cast speed is already marginal - in a very fast-paced game it may make a difference, but in the game such as Pantheon - especially in PvE I doubt it'll be noticeable. First spot at the moment could take either gnome or archai, depending on the strength their passives - gnomish larger mana pool may be more important for additional nukes that he may dish out. Besides I always found silence to be a lot more annoying than mana drain for casters, so there is that. I've planned to make gnome for a start - since they are the ideal lore-wise race for me - all mysterious and knowledge seeking, while I've never been a fan of "(once) enslaved backgrounds" like archai ... And there is also a question about actives. Not to mention, that I wouldn't be surprised if VR either completely revised and changed passives or even resign from them altogether, seeing that many people are not happy with disbalancing from the very start and giving some races edge over the others. We shall see.