I've got two 391 Sportings, a 12 gauge Optima bore and a 20 gauge. The 20 is a Urika-2. The only difference that I can see between the two is a slightly different gas piston that is supposed to reduce breakage. I know several 391 shooters and none of us have broken a gas piston.
The 391 is super reliable. Mines gone 2,000 rounds between cleanings with 100% reliability. In almost 20 years I've probably put over 30 thousand rounds through the 12 gauge and only needed to replace a connecting rod and recoil spring.
My biggest gripe with the 391 is take-down. It's can be a tricky gun to put back together if you don't know a couple of tricks. Seating the connecting rod and getting the bolt in is finicky but easier with the trigger assembly removed. The trigger assembly is easier to remove and reinstall if you push down on the bolt release while doing so.
The forend cap is a pain in the arse. If not loosened when the gun is stored it can seize. I've seen more than one used 391 with channel lock marks on the forend cap because it had to be wrenched loose. Disassembling the forend cap to clean is not easy. Internal snap ring pliers are required and I've got a special tool from Angle Port that makes the job easier.
I've shot several A400s and like them a lot but not so much that I'd trade my 391. The 391 fits me better, the A400 pistol grip is more open and doesn't fit my hand as well. If I didn't already have the 391 I'd probably get an A400.
On the used market I've seen the 12 gauge sporting models in very good to excellent condition for around $1,500.