The BIG difference is in clean tones: they're nice in the original Road King, but nothing particularly special. After it was released, Boogie developed and released the Lone Star, which features the best clean channel they ever created... and thus in the Road King II, Boogie took the clean circuit from the Lone Star and replaced the "average" clean circuitry with it.
So... RK II has superb Lone Star cleans, and also the fantastic Lone Star Reverb unit.
But the RK I still sounds great, and the heavy channels are unchanged.
Read the full details in this review:
http://www.musicplayers.com/reviews/guitars/2006/0906_MesaBoogieRoadKing.php
Scott