Both are a little difficult to acquire today, due to the Dogfathers being out of production and the RD9s not yet back in production because of the Ranger Knives & Ontario deal. I understand that the Ontario Knife Company Ranger Knives line should start being available sometime in September or October.
The RD9 is a bit shorter and a lot lighter than the Dogfather, so it is faster in hand. The Dogfather also has a stouter tip - stronger, but doesn't penetrate as well. Both chop certainly well enough for my purposes, although being larger and heavier, the Dogfather does chop better. It seems to me that the Dogfather is somewhat tougher, especially in lateral stress, but the RD9 is plenty tough enough for even abusive use. On the other hand, the RD9 seems to have an edge more resistant to damage, tearing in particular, than the Dogfather - this is something you would notice only in extremely abusive use. The handles are different, too: Dogfathers have Resiprene-C handles that are pretty good at absorbing impact shock from chopping, but then, they aren't quite as tough as the RD9's Micarta scales, although they are more than tough enough.
Either one will do fine. :thumbup: Get whichever you can get easier and cheaper, or get both.