In my opinion though, if you save up to get a "B" gun, you will end up spending less on firearms in the future as you will have far fewer problems than buying an el cheap. I have a buddy who's gone through three guns in about 6 years and spent right at what I payed for any of my "B" guns. I haven't had one problem with any of my B guns at all. Just my $.02 though.
If you are just getting into the sport though, just buy something inexpensive as you are deciding if you are really gonna partake in the sport in the future.
Enjoy whatever shotgun you decide
I have proven this to myself over and over and over again. Whether it's shotguns, duck calls, rifle scopes, knives, etc., etc., I always started low and worked my way up, losing a few bucks every time. In the long run it's almost always cheaper to spend more money
on an item of higher quality right off the get.
That being said, a properly functioning 870 supermag would be my first choice in a waterfowling pump BUT
it seems like anymore you only have about a 50/50 chance of getting one from the factory so you have to make that decision based on your time restraints. At one time I had an auto that broke down during duck season and I was waiting on parts so I needed a gun in a hurry. I didn't have time to fiddle-fart around with a gun of questionable reliability so I bought a BPS. I prefer the ergonomics of an 870, and if it had been the off season I probably would've bought one and taken the time to give it a thorough testing and working out any bugs.