Two triggers are the most reliable system, since they are extremely simple, and a failure of half the gun generally leaves the other half functional. An inertia switch single trigger system won't switch barrels automatically if you have a FTF, and a mechanical double trigger, while it will, is more complex than two triggers without a switching system.
There's nothing more reliable about external hammers. Internal strikers are going to be far more reliable both intrinsically (they have fewer parts) and practically (they require no manual cocking and can't get hung up on clothing, etc.).
Here's a problem: most cheap double-trigger SxS are made with extractors. That's the last thing I'd want for HD. Automatic selective ejectors would be a must-have.
Now you can certainly get a gun that meets my specs (two triggers or a top-quality mechanical trigger, top-shelf overall build quality -- not
pretty, but as close to 100% reliable as possible, automatic selective ejectors, hammerless) but not for 600 dollars. Once you pass $2000, you can find some; once you pass $4000, you can find a good number. I don't know of any such guns made with 20" barrels, but I'm sure that good gunmakers would accomodate a custom order, for a price.
That's why the 870, 500 and 590 are popular... Even shotgunners who are able and willing to pay a lot of money for their scatterguns, don't want to pay that for a gun that sits under the bed.