late to the party here. This question is so loosely defined that it's impossible to answer, but I'll take a swing.
Interior: Disco seating position is terrible and awesome. The height of the seats in relation to the window sill is superb. Tremendous visibility which helps a lot in pinpointing through tight spots on the trail. However, the seats are wedged in tight against the door, leaving very little room for a sizable driver. Legroom is non-existant, though headroom is ample. however, because the seats are too close to the door, you'll bang your head on the window in any cross-axel situation. The Montero is a lot more roomy for the drivers and passengers, but the cargo area is not quite as usable, (though similar in volume). The visibility in the Monty is good, but not exceptional like in the Disco. However, Disco's develop a lot of squeaks and rattles (i.e., character) which just is not evident in the Monty.
Storage: Disco wins hands down. TONS of nooks, pockets, etc... to hold stuff in the Disco. Montero is much more limited here.
Road handling. Montero is much more nimble, and handles quite well compared to the Disco. Transmission is much smoother in the Monty. Off the line, the Disco's V8 helps move it with less drama than the Montero, which requires more skinny pedal. However, in the 3.5 SOHC engine in the Gen 2.5's I feel there is more top end (passing) power in the Monty.
Off road: The Disco wins hands down. MUCH better suspension design, allowing significantly greater articulation. Plus the transfer case and gearing is much lower and advantageous off road, combined with the torque off idle in the V8, the Disco will walk through obstacles that make the Monty look like the USS Minnow in a 3 hour storm.
Durability: Montero wins by a wide, wide margin. Every single aspect of the Montero is designed more robustly than a Disco. From the interior panel clips to the frame to the suspension members to the wiring harnesses. Montero start aging after 200,000 miles. Disco's are working hard after 100,000 miles and need a lot of ongoing fiddly bits and piece work to extend their lifespan to 200,000+ miles.
Parts: Monty parts are stocked in the local AutoZone or PepBoys or Kragen or what have you. Rover parts are available, but must be "sourced". However, off road components for the Disco are widely available from quality sources. Monty's.... not so much.