TRS texture files are 1024 x 1024, and I'm sure we can make them bigger! Thing with MSTS is that it's old! It may have been realistic back in 2001, but it hasn't been updated like Trainz has(at least not officially). Aside from the route editor, here are the other reasons I prefer Trainz:
-More realistic catenary, and I can make a layout with only some parts electrified, as opposed to MSTS's "all or nothing"
-Don't need to worry about the coupler compatibility
-Can operate multiple trains in a session
-Railyard is a nice touch!
-Session doesn't end when a train derails!
So, ya, I think I'll stick with Trainz!