The 690 tends to be smoother than the 640s. The gearspread is a bit wider too.
IMO, the WRR and DR do a lot of similar things well, but with a different bias. The DR is a tractoring packmule that is smooth on slab. The WRR is a revvy chunk of a dirtbike with lights and a smooth motor. Riding solo on rev-friendly terrain, I'd lean WRR. Riding heavy/2up, and/or in torque-sucking terrain, I'd lean DR. Either bike will do both, but you'll notice a definite bias.