I'd be careful if you know you want to upgrade to the 8. There are many things at play in these iPhone 7 offers and they are not clearly defined.
As I understand it, Best Buy will give you $100 for a 32 or 128 GB iPhone 7 and $250 for a 256 GB iPhone 7 IF and only IF you sign up for a monthly or term lease plan. If you want to upgrade before the term is up, you will have to pay your carrier extra in order to even qualify to trade your phone in for the iPhone 8 or pay an early termination fee. So the phone 7 will be yours to lease for 10 months, hypothetically, and then you'll likely have more than $250 to spend to trade in your iphone 7 (assuming a 24 month exit option), meaning the $250 you get now in a BB gift card is likely not even enough to cover getting you to the iPhone 8, to say nothing of the gift card only being good at BB and not worth anything with your carrier.
Given you seem interested in AppleCare+, I'd think Apple's IUP program would be a better fit in that you have a planned upgrade at 12 months of payments and the coverage built-in.
Many experts on this forum so YMMV.