Adding mashed potatoes for moister/softer cinnamon rolls
Both kinds of potatoes you mention should work as a replacement to a portion of the flour required by the recipe you'd like to follow. I think replacing ~30% of the mass of flour with that mass of your choice of potato is a great start.

The sweet potato might be a better choice because of the added sweetness you'll get. You don't want to go far above 30% if you do that because it will start to dominate the flavor of the roll.

As for softness, it does seem like the end result will be a bit softer although I can't say I've tried it before. You could even consider using some high quality ricotta cheese instead of potato. If using ricotta, I'd limit to ~20% replacement for the flour as this is less starchy than the potatoes. This will help to soften the roll as well.