I have both MSTS and Auran Trainz 2004.  MSTS seems to be more geared toward real world operations (despite the generic signaling) and Trainz 2004 seems to have more of a model railroader type flavor to it although realworld terrain and routes can be modeled with the right utilities and a little time.  Now, in MSTS the route editor and world builder are notoriously hard to use, one must be a software type to figure it out.  Trainz world builder is very easy to use and some fantastic scenery can be done although the track detail is somewhat more lacking than MSTS.  Heck, good HO track looks better.  But, it does give somewhat of a Godlike feeling building routes in Trainz.  Also, all the industries are interactive.  Cars get loaded and unloaded.  Logs, trailers, coal, ore, its all interactive and you see it being done. I've read what Auran's future plans are with the software and they intend to split it in 3 different versions.  They intend to creat a "model railroad" sim.  An intermediatary version which will serve as both realworld and model style, and a full-out locomotive simulation which they hope to market to railroads worldwide for engineer training.  It will be interesting to see what they do with it.