So, I went ahead and purchased these two in a combo (for only $8, I couldn't pass up the SD40-2 pack), and after about an hour of messing around, these are my thoughts:
Marias Pass:

New rolling stock sounds are implemented by default, including brake application sounds

New horn for the SW1500, and nice sounds for the rest of the locos

The route is functionally awesome! Superelevation is buttery smooth (not like even the new version of DP, where its choppy in and out of curves)

Aesthetically, its very nice. Having been to many locations portrayed, I can affirm they did a good job. Extremely good attention to detail.

Physics seem to be MUCH improved. I haven't had a chance to really get a feel for it, but its obvious on the surface. Switching is now fun - the loco brake of the SW1500 is powerful and reacts quickly to input. And you have to really pour the coal to it (or...um...diesel) to overcome static inertia. We finally have a functional SETUP position on the Dynamic Brake control too! Nice!

All freight cars seem to have had a major overhaul in the graphics/modeling department. New trucks and wheel profiles that are much more realistic and scale.
BN/BNSF SD40-2 DLC
This isn't 'just another' SD40-2 addon - its worth the money this time!

Additional freight cars, with some cool loads - and these too have the new trucks and wheel profiles.

Ok, here it is! BEAUTIFUL new cab interior! Finally, notch markings on the throttle, and many other new details.

Sounds are....a disappointment, but easily fixed with BNSFDude's (and Simer4's) awesome modification.

The BN variant is missing some obvious details (as pointed out in an earlier thread so I wont go into it here)





I think RSC really raised the bar with this release. It's another leap up, as Horseshoe Curve was. As I was driving, I kept thinking "this is Railworks? This doesnt feel like Railworks....it feels better."