Well I bought Trainz 12 just because I got it dirt cheap. My biggest problem with it is the bad, jerky framerates I get in trackside camera mode.
I like Run 8 mainly for it's ultra realistic physics and multiplayer. Another awesome feature is linking multiple routes together. It allows you to drive seamlessly from one whole subdivision to another whole subdivision without exiting to load the next route. Considering they currently have the BNSF Mojave and Needles subdivisions released....... with Cajon & San Bernardino Subs, Seligman Sub and UP's Yuma sub all in development..... you could drive continuously all over southern California and across the state of Arizona without stopping. That's a pretty huge development in train sims.
But..... as of now I still play Railworks 90% of the time even with it's flaws. It's the best looking that's for sure. Can't wait to see what TS2015 has in store. They should be releasing info and screens in the next few months. It is that time of year ya know. I really want to know where they are taking it next.

BTW.... Trainz: A New Era is looking good. A little heavy on the HDR and bloom lighting, but otherwise very nice. I like the fact it's 64-bit native, multi-threaded and has realtime per pixel lighting. Definitely gonna need a beast of rig to run with full effects on max at a decent framerate. I hope the new physics hit Run 8's level. That would be sweet!
Hopefully DTG is watching and will bring Railworks graphics at least into the DX10 era.