How much advantage do you get for your extra inch and a half? Either way… is the shorter worth the mag tube capacity reduction? Is the extra length and weight worth the added capacity/sight radius? What are your thoughts for a general purpose defense and maybe food shotty?
I went from a 20" 590 to an 18.5 " 590 and the difference is VERY noticeable. You wouldnt think that an inch and a half would make a big difference but it really does. Now when I pick up my old 20" it feels like a giant trap gun. I put a Hogue 12" LOP on the 18.5" and it now fits into a baseball bat bag for camping trips. I think 18.5" just makes more sense for a civilian. Just practice loading quickly and you wont lose much advantage with the lower capacity.
With the 20" 590A1 most, if not all of the difference in handling from an 18.5" gun I noticed came from the 8 shot magazine tube. I have an 870 Express with the 4 shot tube and I cut it down to 21" (for my girlfriend to hunt with). Even though it has a longer barrel than my 590A1, it handles MUCH better. I’m talking snapping it up and swinging from target to target here. For me, barrel length has less to do with it than muzzle weight. Maybe if I was getting in and out of vehicles alot I might notice the extra length, but I doubt it. We are only talking 1.5" here. I roll with the 20" A1 partly because of the one piece mag tube. If you think you want a higher capacity tube than a standard 4 or 5 shot, take a look at it. The 18.5 ones probably carry and handle a little better, but to me the additional 3 shot mag capacity is worth the extra length. If they made an 18.5" with a full length mag tube that went to the end of the barrel I would be all over it. Mag tubes that are 4 and 5 inches shorter than the barrel are kind of wasteful if you ask me.
I currently own the 18.5in and 20in 870 variants. I too tried the 3+ extension and the 20in barrel . Fully loaded, with a Surefire forearm it throws the balance very far forward, it makes one handed manipulation a challenge for extended periods of time.
Putting the same setup on the 18.5in with a +2 extension, changes the balance drastically (for me). And makes a weapon which has better handling characteristics. For my uses, sacrificing the extra round is well worth it for the increase in handling characteristics.
That may be different for the Mossberg platform. The 20in 870 is an old Wingmaster police model, and the 18.5 was an Express.
Got a Benelli SN Tactical with a Nordic +2 mag extension on it and I can’t imagine having a shorter barrel. For CQB/home defense applications, I think it’s just fine. Sights at that range are useless. Put a red dot on it at that point for target acquisition or at least upgrade the sights to tritium. It’s not skeet shooting. 
I had an 870 with an 18.5-inch barrel and ghost ring sights that held 5 rounds. I got rid of it as I just don’t like the Remington safety as well as other features about it.
I then got a Mossberg 590 with a 20-inch barrel and ghost ring sights and fell in love with it. It also holds 9 rounds of 2.75 shells. Big difference capacity-wise, which was great. But indoors it felt like I was swinging an M91/30. The 1.5-inches of extra length combined with the more muzzle-heavy balance of the Mossy made it feel very different. I am much happier with this shotgun though and will probably never buy another pump in my life. This one fires like a rifle when shooting slugs, very accurate with ghost ring sights.
IMO an 18.5 is great for a house gun. For some reason the 20-inch barrel feels much longer than it really is. Just my experience with the two lengths being discussed.
When I run a SG I run a 14" 870, which I’d gladly trade for a 12".
Something else to consider is the absence of a choke. You mentioned it doubling as a food shotty which I took to mean that you would hunt with it. Your shot pattern will open up greatly unless you are using specialized loads out of a smoothbore. I tried trap AND skeet shooting with my Benelli SN tactical and the results were laughable.
One thing I really like about the Benelli is that the top of the receiver is tapped. I like the option of mounting a rail and/or shell carrier from that location as opposed to the side of the receiver. Tacticalshotgunner.com carries Mesa Tactical products and they offer products with 4, 6, and 8 shell holder capacity at reasonable prices.
I have a Remington 870P 20" with rifled sights.
On my list of things to do is get an 18" Remington factory Police replacement barrel with rifled sights.
I’ve taken Rob Haught’s excellent shotgun class (can’t recommend it highly enough, if you have a defensive shotgun, you need to take that class) and when I ran through the shoot house, that 20" musket barrel was a detriment in close quarters.
Honestly, if you’re state allows NFA items, I’d seriously consider a 14" Remington Police SBS.