The Mossberg 835 amounts to little more than a stretched 500 receiver to handle the longer 3 1/2 inch loads. If your not going to be using the gun for Turkey or Water Foul and want to accessorize the gun considerably then the 500 might be the best route to go. If Turkey or Water Foul are on the menu, the the 835 just makes the most sense.
I recently picked up the 835 Turkey model in factory camo for 219.00 NIB and really love the gun. Yeah I would have rather had the 870 but it would have cost me a heck of a lot more. Despite the 22 inch barrel I did pick up a few limits of Teal and Mallards. Apparently they didnt get the memo that your supposed to be using a 28-36 inch long barrel with a bead instead of rifle sights. I also scored on one fall turkey this season.
I really think it comes down to whether or not you will have a need for the longer 3 1/2 inch loads. Personally since they are both the same price and a gun is an investment that if cared for could last one for decades I would probably opt for the 835 and retian the option to use the 3 1/2 inch loads if desired.