The pump is gonna be cheaper than the autoloader, but less convenient. I'm sure you already know that, but you did ask for pros and cons.
I bought the 870 Express Magnum as my very first firearm. They are heavy duty and high quality. They are also heavy as hell. I now own two shotguns, the 870 and an autoloader (not the 1100). I don't deer hunt with them (I use rifles) but I bird hunt. I always grab the autoloader because, at about the same weight or less as the pump, I prefer the convenience of the auto. Unless you have some type of lemon autoloader, you will not encounter nearly the amount of problems that you are hearing people suggest here. Of all the autoloading shotguns I've used, I don't recall ever having a misfeed issue.
If you have the extra money to spend on this and it's not going to put you in a bind or prevent you from getting something else that you would have gotten if buying the pump, I'd recommend getting the autoloader.