Can't say much about Monteros, but in terms of interior cargo volume, Troopers are larger than even Durangos of that vintage, let alone Explorers.

Reliability is better with Troopers. About the only "typical" problem for those years is valve ticking for SOHC models, and this is more of an aethetic issue than anything to worry about. Lotta people have the high miles you are looking for on their Troopers.

Parts are cheaper and easier to find for a Ford, but on average you will need them more often, too.

Mpg will be slightly better on a Trooper over Explorer, but not good for any of them as you noted...you can get a 5.0L V8 Explorer if you like 13 mpg tops (my friend has one).