As an owner of both vehicles in question, there's not a whole lot of difference between the two, to be honest.
In the Trooper, you get height adjustability in the seat which is cool for those of us with long legs. In the Montero you get the suspension seat which is cool on those rough roads at speed.
In the Trooper, you get two rear doors that can be a blessing at times, but the Montero's one BEHEMOTH rear door makes loading less challenging at other times.
Both trucks featured an underpowered 2.6L straight 4 in some models and a mid-sized V6 in others. The Mitsubishi 3.0 is "better" than the Chevy 2.8, but the 2.8 can be easily swapped for a Chevy 3.4 which has more power and a large aftermarket following.
The SWB Montero is a very capable rig, and the Trooper had a very rare equivalent in the RS.
It's a wash, really. Upgradability with Mitsu factory parts is better on the Mitsubishi side since Mitsu offered some cool options in later generations of the Montero, but the Isuzu aftermarket has a larger following with people still making parts for the old rigs on this side of the ocean.
As for what Isuzu has done...that's a whole 'nuther topic. <img src="/forums/images/graemlins/kewl.gif" alt="" />
Good luck!
EDIT: I forgot to add that I rebuilt my Trooper 2.6 for a large amount of money. I rebuilt the top end of my old 2.6 Raider for a larger amount of money. In both, I used aftermarket parts. When I rebuilt the top end of my 3.0, I spent a lot on factory parts, but they seem to last longer. If you're planning on keeping the truck, do the job right the first time and use quality stuff, or you'll just end up doing it again sooner than later. Just my .02
