Gotta Agree with BigJimP
My younger brother is also a Southpaw, and hated just about every pump he tried other than the BPS, and one other I'm blanking on. He could only find two Pumps that didn't have the ejector pushing the hull into his line of sight, or worse, the side of his face.
That said, I've heard great things about the Wingmaster, and terrifying horror stories of the Express. I suppose the best advice with high ticket items like guns and cars... buy the right one the first time, even if you have to save and spend a little more. Get the BPS (Assuming it fits) for 600ish, rather than several you hate for 200 a pop. Remember this:
The 870 express is often what they give people in Hunter Ed classes where noone cares if you can hit the broadside of a barn, as long as you don't literally hit someone else's barn.
The last words of what passes for wisdom from me would be this: I wouldn't mind shooting skeet, double, or sporting clays with a pump, but single target trap where you usually are only allowed one round in the gun and have to load every time due to range rules... ugh. Give me my O/U every time. A Semi Auto is even better than a pump for single shell shooting. I felt like you had to go through the Marine Corps Silent Drill routine every time you wanted to load the thing. Disappointing for one bang per twirl.