If you're using a slow off-hand, you really want it to be at least as slow as your main-hand. The benefit of having a slow off-hand is that you direct more flurry charges into your main-hand. This is the optimum configuration if you prefer not to use heroic strike as much.
Slow main-hand/fast off-hand means more of your flurries will end up directed to off-hand. This will be desirable if you are looking to maintain a high rate of heroic strikes to negate glancing/proc impale/increase your hit rate. It also gives you an even amount of rage. The other benefit of a fast OH is that you can retain a slow MH and increase the number of times the mob dodges you; this will give you more shots at overpower.
As of the next patch, slow/slow will probably be the clear winner, since you won't have enough talent points to hit impale/imp overpower, and off-hand rage generation will be much weaker.
I've only ever seen a fast MH/Slow OH work in practise with thunderfury.