I'd say Train Sim. Basically because of the wealth of mods out now, and the way the game is.
From what I've read, Trainz is better if your into creating new routes (laying track, bridges, trees, etc)--which from what I've heard is pretty easy. There are also no activities (simulations) in which to operate the trains with real world restrictions, schedules, etc--like you can in MS Train Sim.
Train Sim is about controlling single trains . The cabin views in MS Train sim are far better than that of Trainz, and I would also say that overall, the graphics in MS Train Sim are much better than than that of Trainz.
And most importantly.... theres no collision model in Trainz! So you can't collide two trains like you can in MSTS.