I had a '96 3.1 Trooper, I have not long got rid of it and got the 2.2 DTI Frontera, only because I wanted something newer and less miles.
I liked the Trooper a lot, and would recommend one to anybody. Mine was great on fuel, went very well indeed. It had so much grunt, I hardly ever changed gear, would pull everywhere in top. Nice high clear view of the road.
The 2.2 DTI Frontera is a newer car, it is a lot lower, the driving position is sports car like compared to the Trooper. It has a stiffer ride and corners better- less roll compared to the Trooper. The boot space is a little smaller in the Frontera, because of the sloping back end. The Trooper being straight, with the two back doors it is a lot better and easier in my opinion.
I used to get 29/30 MPG out of my Trooper (I used to keep it around 65 on bypasses etc, regularly serviced, and the roads are a lot straighter here in France), and it was a very easy relaxing drive. The Frontera feels gutless compared to the Trooper. You have to change gear a lot more and rev it more.
I think the 3.1 Trooper has a more robust easier to maintain engine, from what I have read on hear I am constantly waiting for the Fronty to pack up, (and as such I am thinking of selling it again after I re-register it here)