I own both a 870 and a 590.
The 590 has the Vang Comp System and a Knoxx SpecOps Folder
The 870 is by far the smoother of the two actions.
The 590 is much easier to shoot with the compensation and the Knoxx stock.
The safety on the 590 is mounted on the tang and is easier to use than the 870. However, if you install a pistol grip stock on the 590, the safety location becomes a problem..your thumb just isn't up there any more.
A back bored and elongated forcing cone is great for HD and target shooting, but is not so good for hunting. It does make for a very tight pattern but does so by stringing out the shot load. This really makes a tight pattern on stationary or slow moving targets, but on birds or faster moving game, you will get fewer hits because it moves out of the pattern before all the shot has arrived.
If I had to choose one of them, I'd go with the 870 because is feels so smooth and works better with a pistol grip. I use the Knoxx SpecOps adjustable tactical stock because I found the length of pull on the folder is too long for me...and I'm 6'2.
I have not looked into Vang Comp barrels for the 870