Pag Sorento ba yung boto, kupal na?
As with any comparison, the "better buy" for you is the car that suits your needs more. The Montero, being a truck in fancy clothes, has a more rugged chassis that could take more abuse than the car-based Sorento. Obviously, it also seats more because of the 3rd row. A bit more toys too compared to the bare Sorento. Overall, the better workhorse. Not necessarily the better family car, unless you've got a big family that could use the 3rd row seats often.
The Sorento, on the other hand, has a much more potent engine (that doesn't belch smoke whenever you bury the throttle). Also, because it isn't a truck, it handles corners much better than the Montero. Despite the smaller profile from the outside, the Sorento's more spacious whether you're in front or in the second row (the Montero is narrow, and the ladder-on-frame chassis isn't very space efficient). Also, it rides much better (again, car vs truck). Pretty bare though, but fog lamps go a long way in improving how it looks. As a personal aside, the rims of the facelifted version look too small for the body, and I'd really prefer the old one (if only they were still available).
Long story short, if I were to choose, it'd definitely be the Sorento. I would prefer power, handling and comfort over 2 extra seats, toys (that you can add on later) and the truck chassis.
Now, as to why more people buy Monteros, it's pretty simple: not everyone cares about power (you'd be surprised how many people don't even know the concept of horsepower), a lot of people see more seats as better value, and when it comes to SUVs, Mitsubishi has better brand perception than Kia.