I have a S&W in 460 mag and a 500 in a BFR, the reason I bought the BFR was the S&W is just getting the crap beat out of it.
Single action is inherently a stronger action, the top strap could be used for I beams in the Sears tower.
I have seen a picture of a 500 BFR that was BLOWN UP from a squib load and then a full power load right after it. Now when I say blown up I mean in a good way! The only thing that happened was the barrel peeled away from the rear just behind the bullet that was lodged in the barrel.
This would have caused a MAJOR KB in any other pistol IMHO. The cylinder held, the top strap was slightly bowed up I was amazed!
My S&W is nice, accurate but has had the frame replaced, and now jumps cylinders when firing heavier rounds. Like this I'll shoot 1-2-3-click...***? I had loaded 5 in there thinking it was a bad primer (did the normal oh crap safety procedures and waited) nope, the case was the cylinder had over rotated do to the torque causing it to skip to the next round then finally came back around to to the already shot one.
I also like that there is NO muzzle break on the BFR's I got the 10" to put all the possible power out the end of the barrel.
As far as double action vs single action, have you ever TRIED to shoot a quick double tap with your 500 smith? I would end up in your forehead! I can shoot single action just as quickly.
The only plus that I see is the quicker reload. Reloading from the gate is a pain in the rear but considering it's a hunting gun and not a SD gun it really doesn't matter. If I were hunting dangerous game I would have a 45-70 lever as a back up anyhow. So a quicker reload is kinda moot point.
I guess the costs now are about the same for the shorter barrels but as you reach out to the 10"+ barrels the cost of the 500 BFR is really quite a bit cheaper. I picked mine up new for $699
If I could only find a holster for the darn thing