Re: whats the diffrence?
If youre ever gonna do an extended magazine, it will have to be a 590/590A1 or an 835. The configuration of all other 500 series shotguns prevent modification.
Also, most if not all of the sporting guns (at least the sythentic furniture for sure) have pinned forends, and the pinned forends are showin up more and more in the special purpose line. I've seen a couple Persuaders with it.
I wont argue against the 535, but a gun chambered 2 3/4" or 3" will give you many more options in the way of ammunition. The cylinder bore is an excellent choice in general, but I know you've mentioned comp and sport and would consider threading the muzzle.
The 835 honestly offers the most in the way of bein able to extend the magazine, use choke tubes, fire magnum loads and can be modded with most aftermarket parts available.