I might as well toss my hat into the ring! My trainsim collection includes MSTS, at least 2 versions of Trainz, and Trainsim.
I uninstalled MSTS early this year after realizing that I hadn't touched it in so long that I'd completely forgotten how to run it. I'm about to ditch Trainz as well, for reasons specified below. TS 2013 is my train sim of choice.
I'm sticking with TS 201x because RSC has now given me three free major revisions, whereas Trainz wants more money for each new version. RSC provides its official content through Steam, where add-ons can be easily bought and download automatically; independent add-ons and enhancements can be found all over the place and are also easy to install, especially when their creators use RSC's own packager. Trainz has a ridiculously complex add-on system that never, ever, ever worked for me; in addition, most of the add-ons I tried wound up having "missing dependencies" that I could never, ever, ever track down.
As far as playability goes, again TS gets my vote over Trainz. Of course, there's a learning curve with each, but I find TS easier to run overall, especially with the 2013 improvements in the interface (and I do not mean the addition of game controller operation, folks).
Finally, to my eye Trainz looks like a model railroading simulator (especially on its 2D map which is clearly grided in the form of sheets of plywood); many of its so-called realistic animation effects, especially loaders, are a joke, when they work at all, that is. TS, on the other hand, looks like a real-life railroad. I don't mean that it's a perfect reproduction: it is a train simulator, after all, not a train. TS 3013 has brought numerous graphic improvements and RSC is obviously aiming at continuous betterment.
As far as Run8 goes, I wouldn't touch it with a 40-foot railroad tie, partly because of its complexity, but primarily due to its makers' disgusting attitudes. Just to round out the options, I also bought a Subway Sim several years ago. After discovering that I couldn't even get a train to start after rereading the intensely complex directions multiple times, I uninstalled it and buried the box somewhere.
Best of luck and I hope you enjoy whatever you choose,
Tom Pallen