I have a new BCE and am waiting to see how it treats the WalMart and Target i frequent for points. My Barclays treats both as grocery, so double points. If both are grocery for Amex, I would think of PC to the BCP. If not, I will continue to use the Barclays at those stores. Gas is double on both BCE and Barclays, but Barclays doubles "utilities" (cable tv, cell phone, water).
It specifically excludes superstores and warehouse clubs. I doubt if all WalMarts are superstores, but I guess all you can do is buy a small amount at your local store and see what it gets classified as. The BCE grocery cap means your max benefit is 180, and on that spend Barclays would give you $120 (and if you spend more, Barclays keeps going) so it's not a life-changing amount of money!