Personally, I preferred Trainz. What kind of Steamers does KRS have to offer besides those British ones? Any 4-4-0 types? Actually, from what I have seen, the graphics are better than MSTS, and about matched with Trainz, depending of course on the fact that Trainz is well established as well as the creator's putting their minds to it.
The thing with MSTS, route editor aside, is that it's old, and hasn't been updated, so the game's capabilities are limited. Trainz has been updated quite a bit, and continues to improve. That gap between MSTS's release and KRS's has given KRS a boost so it's top of the line.