Both vehicles have a maximum gross weight of 7500kg and both are 4x4. However the Unimog has 20 inch wheels (rather than 17 inch), twice the ground clearance (with portal axles) a 51° departure angle (rather than around 20°
, front as well as rear differential lock, a raised air intake (for fording rivers) a pressurized transmission system (for fording rivers), disk brakes all round, a parking brake that will hold on a 45° slope (if you dare!) etc. etc.
It is also worth noting that the intended load distribution of the two vehicles is very different. The Unimog, like many true off-road vehicles is designed for a near equal load on each axle, whereas the 815D is intended to have substantially more weight on the rear axle (which consequently has double wheels).
But the Unimog also uses considerably more fuel per kilometer on the highway (say 70% more?) and is far less pleasant to drive than the 815D. The U3000 chassis/cab probably weighs 1000kg more than the 4x4 815D. Most importantly it costs a lot more.
In very round terms I believe that a 2x4 815D chassis/cab costs 45,000 Euros, a 4x4 815D chassis/cab costs 62,000 Euros and a U3000 Unimog costs more than 100,000 Euros!
In my opinion the extra cost of a 4x4 version of an on-road truck (if available) is worthwhile, but the extra cost of a true "off-road" vehicle is not.