If you are comparing TDI with 5-speed to V8 with auto, it is not better for slow off-roading.This debate is nearly always divided by intended use into several camps. The results resemble this:
- Primarily off-road
- Slowly: TDI
- Fast: V8
A single giant advantage of a TDI is fuel consumption.
In one long driving day, Chris's 300TDi/5 speed manual drank up 8 gallons of diesel fuel, while my D1 V8 with auto box - 16. It is the closest tit-for-tat comparison that I know about, it was slow enough that aerodynamics of either truck mattered little, and the terrain and pace was exactly the same.
I could guess that with a five-speed the V8 could've only taken 12-13 gallons instead of 16, but that's that.
There is a marginal advantage of slightly-better waterproofing of TDi compared to the V8, but that's solvable.
Reliability-wise, I don't think there are any advantages.