Offhand speed has no impact on total rage generation, it just makes it more consistent with a slightly faster OH. On the other hand, you do get more Whirlwind damage out of a slower one.
In my rather extensive experience, it doesn't matter much in terms of single-target damage, although obviously it's a bigger difference when AEing; but the difference between a 100 DPS 1.5 speed OH and a 2.6 speed OH is ~12 DPS after factoring in DW spec simply because you don't Whirlwind that often. Look at the OH's stats and DPS first, and look at the speed last.