I am still getting familiar with my 870 from Big 5 and have no personal knowledge of the Mossberg 500, except that most owners are happy with them.
As for the Big 5 special, while I believe they will sometimes offer a different package, the one you are talking about is probably on sale once a month. Do NOT rely on what any Big 5 sales clerk says. I have never seen a 28" barrel that took screw in chokes to be included in the combination package. It comes with a fixed modified choke. The other primary difference between them and a standard Mossberg 500 is the action slide. The standard slide consists of two main pieces ( a metal tube with action bars welded to it and fore arm held on by a nute). I understand that on Big 5 special uses a Marverick action slide (Maverick is a Mossberg company) that has the two action bars molded into the fore arm. Some people say it rattles more then the standard and some don't care. I think the choke is the big issue.