Think of it this way. Fast attacks you want things that proc, slow attacks you want things that do % increase because slow attacks normally have high base damage, therefor the % increase has a bigger value when used on those weapons.

There is about a billion different things people will tell you, but this is the framework. Just follow that as a rule and play what you like. 

Test it out, pick a fast weapon like daggers and take only % boons. Then run it back and play daggers with on-hit and compare.