The Patrol is a far better choice for offroad than the Pathfinder.
You have access to parts from anywhere, and I mean anywhere and you will find parts are cheaper because so many produce parts for them. They are like the standard model everything is based on.
The ZD30 engine is the better of the smaller engines but it is not as good as the old 6 cyl 4.2 engine. As Kelvin said the drive chain is almost bullet proof and they will last for many years.
Remember though the Patrol is better off road and reasonable on road where as the pathy is more the other way, reasonable off road better on road.
So it depends greatly on your choice. if you want a great touring car with unreal offroad ability go for the Patrol. If you want to just tour and do the odd bush track not too extreme stick with the PathFinder.
Note: Make sure you get history on the Patrol, check that it has had regular services etc etc. Also I would be asking why some one would want to swap the Patrol for the Path Finder. Maybe they just want to downsize which is fair enough. But I would ask.