While leveling as fury...

[Zakkon]

Should I level with a fast, or slow OH? I was thinking slow because fast might make you miss a whole lot more.

[Jaska]

Fast will miss more often, true, but it will also hit more often, and it will have a greater consequence when a slow weapon misses. Also, a fast offhand will be eating your Flurry charges, but on the other hand, that may not be an issue if only you crit often enough.
Consider mainly how much you want your WW to hit for (slow OH) and how much rage you want to generate (fast OH). If I were you, though, I'd go for a slow offhand, especially when I'd get TG.