I believe the difference is the finish.
When I worked at a Gander Mtn. a couple years back, IIRC, you could get the Express barrel (parkerized-type finish) for about 65% of the cost of the Wingmaster (bright blued finish). Both were rifled and one should fit the other's receiver. In fact, I have a Wingmaster barrel just like the one you're after, mounted on an Express gun. So they're definately interchangeable.
I've heard vague things said also about the steel being different from the Wingmasters and the Expresses and maybe they are, but in real-world situations, I don't think it matters. Either will do the job.
Also, I don't remember exactly whether or not Remington offers a cantilever barrel with a smooth bore. I don't recall ever seeing one but that doesn't necessarily matter.
Good luck in your search.
Oh yeah, you might also want to check after-market vendors, too. If you can't score a Remington, I believe there are Hastings, Mossberg, and Ithaca after-market barrels available for the 870.
*edited to add: If you find one that you want, make sure the barrel ring is the same style as your original barrel. Depending on the age of your 870, you may or may not have a barrel ring with a little ball detent in place.