Urea vs Non-Urea.
Urea Wins.
Urea based systems work the following way:
VW non -Urea system:
Oxidizing catalyst (like older TDI's) is mounted ahead of the DPF, this provides an initial scrub of Soot, HC, CO and reduces the load on the DPF. This is also installed on the VW small vehicle DPF system. The Oxidizing catalyst is a direct pass catalyst which does not require monitoring for restriction (back-pressure), each passage is open ended and exits at the DPF substrate entry point. Higher loads on the motor increase the efficiency of the Oxidizing catalyst, but only to a point hence the need for the DPF.
Any remaining PM, CO and HC is either trapped or burned off as the gasses pass thru the heated substrate of the DPF. Each passage is a dead end channel This forces the PM to accumulate until being burned off in the filter. Gasses are broken down further as they pass thru the filters substrate to the exit passage leading out of the DPF housing. Since the only gasses can pass thru the substrate you have a near perfect filtering of all solid matter.
By the time the gasses exit they have nearly eliminated all PM and resulting gasses. However because of the lean burn of the diesel it is not possible to fully reduce the NOx hence the SCR.
As the remaining gasses enter the SCR urea is injected based on inputs from a NOx sensor at the exit of the SCR system. The SCR pump will inject the fluid as needed to insure the NOx is completely eliminated out the tailpipe.
Audi/MB AdBlue urea system (first module is the Oxidizing catalyst ahead of the DPF).
The result is Soot free exhaust comprised only of CO2, NO and water.
After all the miles we have put on our SCR cars, the tailpipes remain soot free and appear as if the engines have never been started.
If I had a choice, SCR all the way.
The VW system requires not one or two burn-off cycles but three. Each catalyst requires a separate and individually controlled burn-off cycle thus the hit on fuel economy. In contrast an SCR system only requires a single burn-off cycle which is further reduced with the coexistence of the oxidizing catalyst.
Urea in the ML/GL consumes about 1 gallon per 1500 miles. Consumption is a direct result of fuel consumed and average load. Given the 24 city, 31 hwy fuel economy of both vehicles and their weight the cost of the system works out to about $30 per 10,000 miles in urea cost. If you figure that urea costs about 7.5 gallons of diesel fuel at current prices, I suspect that SCR easily saves 7.5 gallons by virtue of eliminating two additional burn-off cycles over the course of 10,000 miles if the VW system were employed.
Another benefit is the requirement of only a high pressure EGR system where VW requires both a high pressure and low pressure system. The low pressure system despite it's soot free introduction of gas results in increased moisture into the intercooler where the charge air and gasses are further cooled for entry into the motor after passing thru the turbo's compressor.