Hi Ed,
I've never owned a Wingmaster, but have a BPS and an Ithaca 37. Both are bottom load / eject.
Comments:
Love 'em both. I've read that the bottom load / eject is slower to load, and can be a bit of a pain for trap and skeet for that reason. But balance that against the fact that the empties drop at your feet. You don't have to chase them. Not a factor for me as I don't shoot clays, but maybe for others.
The BPS is my pheasant / chukar gun, and the 37 is for ducks and geese. I grew up on doubles, and to me, the only place for a safety is top tang, like the BPS. On waterfowl, you have more time, so finding the safety on the 37 is not as rushed. But when a pheasant busts out of cover, I want the safety right under my thumb.
For me, that was THE deciding factor when I bought it.
I've also read that the BPS is a pain to deep clean, but mine's only 3 years old so hasn't needed that yet less than 1000 shells through it. If I have to send it to a gunsmith once a decade for a deep cleaning, then so be it.
The flat sides of both guns seem to keep out junk, rain and snow better than, say, my buddies' 1100's.
You'd also want to compare the weight. BPS is no wand, and the 870 might be lighter if that's a factor for you.