Looking to upgrade my wife's iPhone 6 to a 7 plus or 8 plus. I generally buy used phones if they cost enough less than new. Is this a good strategy? Second question is if 8 plus is worth the extra cost. I looked over the specs and many of them are the same. She enjoys her iPhone but she's not a power user. Likes the idea of a larger screen mostly. Also looking for any suggestions as to good places to buy a used phone. We use Consumer Cellular which is AT&T.