I've had a Benelli nova since just a couple years after they came out, and own a rifled slug barrel made by the older Ithaca company. It's a fine shotgun, but not one I would recommend as a slug gun. The biggest reason is Benelli barrels are expensive beyond any reason, think $400+, don't have much of a used market, and Longbow is in Canada.
I never could run my Nova like I could a 500, probably from the long 3 1/2" stroke. It is tough as nails though, same as any other quality pump action. I haven't shot the slug barrel in some time, my Nova has been my main waterfowl gun for 15 years or better now, and I can't remember ever having a problem.