870 hands down, especially over the 535, for what you want.
The 535, no matter how good it may be, is not a common gun. Barrels, chokes, stocks, etc. are all much harder to find, sometimes impossible to find in the configuration you want.
If you want a multi-purpose gun, the 870 wins hands down, no comparison.
For the 870, many vendors make high-quality aftermarket parts (e.g. Hastings Paradox slug barrels with sights or scope mounts, with rifle-like performance), and a huge variety of configurations (e.g. stock sets from Ram-Line, Cabela's, Hogue, Boyd's, Wenig, B&C, Remington, Knoxx, ATI and more, including folders, Monte Carlos, thumbholes, every kind of camo, synthetics, walnut, laminate, even high-end customs). You can find a lot of it on eBay, too.
For the 535? A few barrels, from Mossberg. That's it.