If I want to DW, is it better to have fast or slow weapons?

There are points that speak for both of these. Let's have a look at the main hand first:

- Slow main: Weapon-based attacks like cleave, whirlwind and overpower do quite good damage. Choice of many, PvP preference.
- Fast main: You can spam cleave and HS much faster when you want to dump rage. And since you'll do lots of attacks, your enemy will dodge often, thus you can overpower a lot. PvE preference if you're not scared of overnuking a lot.

Keep in mind as well that your offhand does only 50% of its damage (up to 67.5% with talents - 50% * 125%), so you should probably have the higher DPS weapon in the main hand if they're not equally good.

For the off hand, we'll need some math first. We crit equally often with the main hand as we do with the off hand, same for specials. So whenever we have a main hand swing, we look at the last 3 swings and look if any of them was a crit. Since all kinds of swings have the same crit ratio, the equation for that is:

1 - ((1 - crit%)^3) = 57.8%

So this speaks for the off hand speed not being important for flurries, and we should have a look at the other points below. It is actually correct that for white damage alone, the off hand doesn't eat away flurries at all, as it produces exactly as many flurries as it takes away.

However, there is one thing we forgot so far: Instant attacks. These occur between hits, and with a slower off hand, it's more probable that they are closer to the main hand swings, thus giving more flurry charges to the main hand and not the off hand. If we use a special, that means that for the next 3 swings after an instant attack we don't take the 3 previous charges, but actually the 4 previous ones into account for our flurry calculations, making the flurry probability 68.3%.

So we can sum up, and add some other points:

- Slow off: Slightly more flurried main hand hits, bigger frontload in PvP (before your opponent moves away and you can't hit him anymore anyway)
- Fast off: More rage from Unbrindled Wrath, more overpower possibilities (since your enemy has a per-hit dodge ratio)

Generally though, speed doesn't matter that much. All those points are minor compared to the dps and stats of our weapons.