Its a matter of price and finding what you want. Disney may have BCV add-ons (they come and go) but I am not sure of price but believe it is low $100s. With Disney, you pay no closing costs for an add-on.
Resale (and Timeshare Store is a good group), you need to consider price plus closing costs to compare to Disney price but should be cheaper. Then, you have to find what you want. For convience, you probably want to try to get one in own use year If you do, Disney will treat it like it does any add-on if you ask them to set it up in system that way; i.e., points will be all together for banking and use purposes. If you buy a different use year, contracts will always remain separate; e.g., when you reserve using both contracts, you will need to reserve some nights with one contract and the rest with the other and then link the reservations. Not too much of a pain but there is a little extra work keeping track of two use years.
Also, before buying you want to make sure of status of points in the resale contract, e.g., a contract that has no current points and next year's points are already borrowed and used is worth significantly less than one with all current points and maybe even some banked ones.