Zone and BDS are sister companies both owned by Fox. Their kits are ROUGHLY the the same. If you want to get technical, BDS comes with top of the line struts and shocks, whereas with Zone those are add-on options. BDS also has a no questions asked warranty on all of their parts, Zone has a limited lifetime warranty.
If you're going by kit, the BDS 4" and 6" kit are the same. The only difference between the two are the height of the steering knuckles, and the spacers (to keep stock geometry). So really, same parent company, roughly the same warranty, and the same parts (with the exception of BDS's struts and shocks).
And obviously the price. BDS is about $800 more expensive.