Your questions are fairly broad and will take a fairly long explanation to answer thoroughly, but would not be that difficult to answer yourself if you do some reading on this site. The Montero was built on it's own frame where the Montero Sport was designed as a slightly lower-market vehicle and shares it's frame with the pickup. They do share a number of components, but in general the full size Montero is considered to be a bit stronger and stouter. Either can be modified to whatever extent your wallet will permit, but neither has large aftermarket support for off-road parts and accessories. Thankfully the Monteros are far more capable and durable in stock form than most modified Jeeps with $5K worth of suspension and drivetrain modifications.