Proc depends on the situation. Speed also depends on the situation, and where in the style chain you are at. I like to use fast mainhands when I NEED to get a style effect off, such as a snare, ASR, etc.
Style-chain dependent:
I START a style chain with a fast mainhand (usually in Right Hand slot). I don't worry about not having a big hit for this first style because the Growth Rates usually suck for the first hit in a chain (there are a few exceptions.) Because the speed of the weapon along with haste/quick should have me near cap speed, I will swing the next weapon hit at cap speed, even if it's a slow weapon. The 2ND (and the occasional 3RD) hits in the chain I use the slowest possible weapon, usually stored in the 2hand slot. The 2nd and 3rd styles in a weapon chain usually have excellent Growth Rates, 0.88 and higher. This is why you want to land these styles in the chain with the slowest possible weapon.
So in summary, fast weapons just to land particular style effects, and fast then slow for style chains.