I'm torn between the M2 and the Defender. Seems the 2 give and take from each other. What one gives the other takes. This comparison is for the same size 60x25x31.
The M2 is new and no frills. The M2 has 6 active locking bolts, 4 active on open side, 1 active top, 1 active bottom, and 4 fixed on hinged side. The M2 has no fire protection in the door other than the 3/8" plate steel door. M2 has 3/16" body. M2 Fire rating is lab tested for 50 minutes @ 1450 degrees (equivalent to 60 minutes @ 1200 degrees).
Now for the Defender; 11 active bolts (4 each side, 2 top, 1 bottom), Fire rating is lab tested for 90 minutes @ 1680 degrees. The defender does have a smoke seal also. Body is unibody 10-gauge steel and Door is overall 1.75″ thick with 1/4 inch steel plate and 10-gauge steel.
So I see the M2 has more steel, less bolts, and less fire protection and is about 3-400 cheaper.
I'm leaning towards the M2 mainly because of the recessed door giving about 1.5" more clearance in width. I measured them myself and even though the website says they are the same the M2 is narrower front to back to get it in those narrow closet doors.