MSTS was better when it came realism, because it was simulating real trains and the way the textures were done was awesome.
But, Trainz is the best because I like where it simulates model trains and because of the ease to reskin, build routes, the graphics capability (Can be better than Railworks graphic-wise), and I liked the fact that it was easy to start building right away. MSTS was difficult, because I could only drive and I couldn't build routes, let alone installing content was hard.
I like Trainz a lot better because of its potential and ability to have endless scenery and easy installation.
Trainz wins hands down.