If money is a concern, and you want something more recent that could not possibly be questioned, try the new Donner Pass route from TrainzItalia. If money isn't issue and you like the idea of driving thousands of tons of steel through pine trees, try the less recently released Laggan Sub-division from Roy's Trainz.
That all said, I think others here have said it. The Content is what makes MPX worth the money. I've heard from a few people that live near the actual Marias that there is yet to be a single Route in any Simulator that has done the area justice according to them, but then, living there, maybe they're nitpicking? I wouldn't know as I've never been there.
Either way, won't know for sure unless you buy it. Just tell us your impressions later if you do.
Falcus