My 1301s I've got have all been more reliable than my 870 (express I bought about 10 years ago and have upgraded a ton of internals to essentially make it a P model). In about 1200 rounds through the main one I shoot I've had one malfunction... It was toward the end of a class in June and it was bone dry, so I'll let it slide.
As much as I love the M2 (I had one for a couple years) and didn't have any notable issues (granted I didn't use it like I use the 1301 currently), I wouldn't trade the 1301 for an M2. There are factors that can affect inertia guns that don't seem to be present with gas guns. Adding things (side saddles, lights, etc) has negatively affected reliability of the M2 (not on every gun, but it has happened). I've also read of people having issues when using push /pull with inertia guns. Plus the 1301 is less expensive. To me personally I feel like it is a no brainer.
My wife, who is fairly recoil sensitive and not TOO into guns, took a 3 hour shotgun block that I taught and ran a 1301 like a champ. With the exception of about an hour of dry fire prior to the class, that was the extent of her shotgun experience. The learning curve is very short. No doubt she ran it infinitely better than she could a pump gun. If I have the option, I'll take the 1301 over an M2 or 870 any day.
Also, I've trained a LOT of shotgun with a lot of different instructors, and Chris Fry's class might be my overall favorite.