A few dismissive posts above, but as someone who seamlessly moves between Railworks, MSTS and Open Rails, I can see the advantages and disadvantages of all of them. The real big upside of Open Rails is that it will run the many very high quality addons for MSTS as well as stock, in particular, engines, designed specifically for the sim.
One example is Bazzas DRGW engine on Gtrax DRGW for Railworks - designed, originally, to run in Open Rails. What more need be said.
Open Rails also doesn't need a state of the art computer to be able to run it at high spec. Don't knock it until you've tried it - and by trying it I don't mean a quick 5 minutes - that is only enough to realise you can't use the same controls as Railworks.
All in all - I like RW, MSTS and OR (not that keen on Trainz)
RW has good visuals, but poor simulation, works differently on different systems and sometimes refuses to work at all - also tied into Steam which can produce its own problems. Potentially very expensive if you are continually tempted by DLC.
MSTS is old, but on a good rig (or even a half decent rig) produces good simulation, good visuals, apart from cruciform trees and foliage, and even these can be sorted out with skilful placement and modelling. Ground texturing can be good and very accurate - also high resolution, unlike RW low res ground textures.
Open Rails takes all of MSTS one stage further, with in game changes to weather, lighting etc. clickable switches and uncoupling etc. AND it is free and being supported by volunteer enthusiasts who are not in it to make money - but who want a good quality, workable simulation of trains with good visuals - seems just about top of the heap to me :)
Check out visuals - 2 from Open Rails then MSTS below. I don't have many shots from OR, but it runs what's below within MSTS perfectly, with good physics and visuals....
Click to see full size.
Open Rails


MSTS





All three, RW, MSTS and OR staying on my rig :)
Rod