Can't decide, which would you keep?

I'd keep both. They serve very different purposes. The combat rifle is my preferred mid-range weapon. The combat shotgun is my preferred weapon for close range combat and particularly for ghouls. I don't think a direct comparison between the two is very logical.

edit - If you're going more for carrying a single gun at a time or something along those lines though, the combat rifle is a much more versatile gun. The combat shotgun doesn't really have much range. The combat rifle on the other hand is great for close range and mid range combat, and can even serve as one of the best sniper rifles in the game if you put a scope on it.

two saves, one has the shotgun, one has the rifle, can only have one

If it were me: wounding combat shotgun, though I am very biased to this weapon because it's a perfect closer. Sweep with the rifle, close with the shotgun. Win-win.

Gotcha. That changes things then.

Personally, I'd keep the explosive combat rifle and I'd make it semi-automatic instead of full automatic.

I would go rifle if you need the range, or the shotgun if you are a more up-close character.

The shotgun does so much damage with that mod it will easily break your game and remove any challenge.

Go with the rifle.