Well,that is a very interesting question for me too.I have got both Sub Command and Dangerous Waters,but still decided to go in for Sub Command with SCXIIC.
The reason of that was,that I really wanted to play a
pure submarine simulation.
And Sub Command with SCXIIC suits all my needs just perfectly.
Dangerous Waters is a great simulation game either,but it is already a
mixed one,while being purely naval,though.I am not very fancy of driving a frigate,or piloting an "Orion"-I just love submarines.
So,I think,that there is no better choice for the fans of modern nuclear submarines,than Sub Command SCXIIC.