I would seriously look at Mossberg. I have a 500, so it doesn't shoot 3.5" shells. However, you can get a 535 which is a 500 that can shoot 3.5" shells. Mossberg also has the pump 835 Ulti-Mag and the autoloading 935 if you want to go that route.
I've handled the 870 a few times, but never shot one. I haven't even had my 500 for that long; only for about a year and a half. Personally, I find the controls of the 500 more natural. The safety is a tang on the top of the receiver so it's easy to reach, especially if you shoot left handed. The slide-lock release lever is right behind the trigger so it's quick and easy to reach. I've heard the elevator can pinch you when you load shells with the 870. I can't speak for that, but with the Mossberg it won't pinch you. The receiver is aluminum, however the bolt is steel. When you lock the bolt in place ready to fire, all the pressure is directed at the bolt, not the receiver. The 500 is super easy to take apart for cleaning. There's some videos on YouTube, but once you do it a time or two it's a snap. I love taking the 500 apart.
I know the 500 can't shoot 3.5" shells, but like I said the 535 is basically a 500 that can. My 500 came as a bundle with a 24" rifled barrel with scope mount, and 28" choked field barrel. I also bought an 18.5" barrel for it.
I've also read good things about the 835 and 935 if you want to go that route. You can't go wrong with any of the options you're looking at. If you can, check them all out and choose which one you like the best.
