I have a Benelli Nova black 26" barrel. My intent was the same as yours, have the stock barrel for most uses, then get a slug or rifled barrel for deer hunting. However, I soon found that with the price you have to pay for a new (or additional) Benelli barrel, you're better off just buying a new gun altogether.
What should also be mentioned is Benelli's customer service. When I bought the gun, I found that one of the chokes wasn't properly threaded and wouldn't screw into the muzzle. I called up their customer service line, explained the issue, they said "no problem," and promptly sent my new choke 1 3/4 YEARS
later. I actually forgot about it until it showed up (this was after calling about 3 and 7 months after I had made the initial call, to which they told me my request was still logged and the part was on order). Uh what? I'd hate to know what the wait would be like if something major happened.
That said, the fit and finish of the Benelli is far superior albeit mostly plastic. I wanted to buy American, and was initially going to get the Remington, but its finish and especially FIT is pure crap. It is absolutely despicable, and the owners, management, and workers responsible for it should be ashamed to put the Remington name and American stamp on it. It wasn't always this way, go find a nice used 870 Wingmaster from the 80's and grab the fore-end grip - give it a little shake - nice and tight. Look at that finish - BEAUTIFUL! I would be honored to own one of the this, and intend to.
Now go and pick up a new 870. The finish is garbage. Look at that beautiful plastic trigger! Give the fore-end grip a shake. "Wait is this thing attached properly? Here, lets try another one. What the hell?" It rocked from side to side so much, I thought I was going to break it off from the leverage in the amount of space that it moves (probably could if I didn't treat it like a baby in the showroom).
Obviously, the 870 and the Benelli have a reputation for reliability, but is that the
ONLY factor in this game? Personally, I don't think so.
I'm sure Remington's customer service is just fine. There's also no question as to the accessories available for the 870 - walk into most sporting goods stores, and you'll find stuff for it.
In the end, knowing what I know now, I wouldn't buy either. I'd either find a very nice used 870 from the glory days, or save up and buy a premium gun from someone else.
Hope that helps.