MJay - defensively, and with proper (heavy enough) loads I think it will be exceptional. Only possible detractor is lack of chokes - depending on your load choice and expected ranges, that may not be an issue at all and you may prefer not having a threaded choke that could back out. And if you like ghost ring sights, those are a more expensive option on the gun so add to its value.
If you have any cycling issues, consider getting the field (straight) stock, or just get familiar with where the pump switch is - it can be used quickly and naturally once you get used to it. But as long as you're not trying to run too light loads or hang too much stuff from the gun, it should work great. I also have field stocks for my Benellis, although I've left the PG on the M3 just because it's cool
. My M3 also came with the factory light mount, but you can do the same with the tube clamp mounts that are available - as you've already got a Surefire fore end, you obviously like the mounted light, but as you know there's not one available for the M3 (unless they've started making one recently). Plus side of the M3 is being most likely used on semi auto, there's no issue with the light or switch wire hanging or hanging up with the pump action like if you mounted a similar setup on your Mossberg.
So - great gun, extremely reliable with proper loads, can go high round count without cleaning if need be (many gas guns would choke trying to shoot a similar number), and lots of cool features.
On the proper loads part: some Benellis (my M1 one being one) will run with absolutely any load. It eats the cheap Wal-mart bulk pack stuff all day, which is a good test for a semi. It wouldn't do that until I lost the PG stock, though (it would run heavy defensive loads). That's where my M3 is - it's not reliable with light loads, but I've got the PG stock on it at the moment. Don't care as I can be on pump in a heartbeat, and this is an awesome gun even on the pump setting. Now YMMV - I have a tendency to lean into a shotgun more so I can increase my ROF, and that's counter to what an inertia shotgun needs to function - you may never have an issue with reliability. I think it WILL work reliably with heavier loads, and MAY work reliably with lighter loads depending on the particular shotgun and your shooting style. Just make sure you've got a good load in it for you before depending on it defensively.
ETA: MJay's comment about getting an M1 just sunk in after I hit submit. Also an option - counter to what JTWO did, I kept both
- my M1 with a Surefire is my go-to defensive shotgun (although I'm more of a rifle and handgun sort of guy), and my M3 is there as part of the collection. Of the two, I started competing with the M3 first, then went to the M1 once I got one of those (my current competition shotguns I'll save for other threads
). As noted in my first post in this thread, the M1 is lighter, smoother, more reliable, has chokes, and a Surefire fore arm. The chokes were a significant part of that choice as the heavy steel plates at longer ranges in some matches were simply too much for the open-choked M3 to handle. But, if the switch action interests you, I think the M3 would also provide exceptional service provided you verify your loads (as you should do with any firearm).