if you 'mostly' drive on rubble roads, dirt and gravel roads, washboards and field type environment and you -do not- carry large hitch or cargo loads; the 5100 will be the best possible ride for that type of use.
If you carry a lot of equipment, tools, gear or anything above 400lbs of cargo, the 4600 are more tuned for your use but wont be as soft and comfortable of a ride on the rubble and washboards. 4600's have been experiencing stock-out situations though, so check with your suppliers. Stateside we are back ordered on 4600's til October, and that's for orders placed today.
The deciding factor between the two shocks tends to be the traveling load, if loaded down the suspension will naturally cycle more with a softer shock.