It depends on the kind of off-road you are planning to do. We picked the Dakar because we wanted to cross Africa and possibly go through tough roads in South America. In that case, the higher ground clearance, the longer suspensions, the 21" front wheel and the possiblity to add the 39 liters tanks from Touratech were a decisive factor.
If you stay on an easier kind of off-road (no big dips and bumps, shorter distances), the normal 650 will do. We got to try it once and it felt much easier to drive - nimbler and lower than our big Dakars. We had much fun using it, slaloming between cars like on a scooter.