I used to own a Nova. I liked the gun well enough, just not enough to keep it when I had better guns. I looked into buying extra barrels and concluded it was more cost effective to just buy a 2nd gun with the Nova. I see used Nova's selling for less than a new extra barrel.
I actually like the gun a little better than the 870 and a lot better than the Mossberg 500, but if having multiple barrels for the same gun is a priority then either the 870 or 500 is a better choice. That was a big reason why I sold the Nova and just kept the 870 with extra barrels I already owned.
I'd be on the lookout for a used model with the barrel I wanted. I think you'll be better off. One other thing to consider. When I was looking for extra barrels I found several sources that listed extra barrels, but never one actually in stock. I'm not even sure you can actually get one. I don't know anything about the Carlson barrels