As far as the build quality goes the TDCi's I have driven have been good, probably the best rovers i have driven for comfort, long periods driving in fine bull dust in africa and not a lot got in, had some issue's with picking the right gear to be in in some circumstances, breaking crust on dried-up river beds, but that would probably be overcome with practice. The TD5's seemed a little less affected by the harsh enviroment and less prone to niggly faults, also easier/cheaper for servicing.
If someone else picking up the bills TDCi, if it comes from my coffer's TD5 on balance.
The 200 and 300 TDi's under the same duties seem to keep going on and with less down time, a good well fettled TDi would be my choice if it's going to be harsh, in the UK the one with the best heater and least drafts would be a strong contender! (TDCi).