The B-stock you are asking about (typically sold through online outlets like Audio Advisor, Music Direct, Acoustic Sounds, Needle Doctor.com) are indeed cosmetic flaws that do not have anything to do with functional performance of the table.
I would suggest calling whichever retailer you are planning on buying from and asking them what specifically is the cosmetic issue so you can assess specifically if you are ok with it or not. If ok, then with the $400 savings (actually I would still try negotiating down a little bit more), I would upgrade from the 10X5 to the 20X2 and upgrade your phono down the road when funds allow. Your 640P should easily allow you to hear the sonic upgrade in going from the 10X5 to the 20X2 (more resolving, more dimensional and more articulated and deeper bass).
I would suggest calling whichever retailer you are planning on buying from and asking them what specifically is the cosmetic issue so you can assess specifically if you are ok with it or not. If ok, then with the $400 savings (actually I would still try negotiating down a little bit more), I would upgrade from the 10X5 to the 20X2 and upgrade your phono down the road when funds allow. Your 640P should easily allow you to hear the sonic upgrade in going from the 10X5 to the 20X2 (more resolving, more dimensional and more articulated and deeper bass).