Learn to use the rifle really well. I can use it in any situation. In some cases using the SMG will have some advantages over the rifle, but if you can use the rifle really well, you should be able to beat SMG users and Shotty users without too much trouble. However, if you are quite good with the rifle at mid range you will be amazing with the SMG at mid range. The reason is that the rifle is the cleanest form of shooting. One shot lined up on the head. If you can do that with the rifle, you can do that with the SMG with 3-5 shots.
However in extreme cases I will use the shotgun (ambushing, arena).