I think it depends upon the size of your room really. We got the 7.1.4 and have a 20x20 family room that this system is in. Sometimes I wish we would have went with the 2 sub system since it doesn't seem to be quite enough. When we finish our basement this winter we are going to go with the 9.2.4 because we are for sure going to need the 2 subs down there.
I guess I would say go with the 2 if you are worried. They have said that you can't add another sub if you go with the 1 sub system and want to add another.