definitely man if you even want a cheaper 6s, i recommend you getting it from apple. i personally do not trust third party resellers because of their quality checks, they might have hidden problems. also keep in mind that new iphone is coming out in few months time. if you like performance on your phone then suggest you wait it out until the new phone comes out later this year (may be you could save and get the new one, its expensive but will be worth the wait especially an upgrade from iphone 5, you). also apple doesn't allow you to downgrade to older version of the iOS because apple, so if you want ios 11 then there might be performance issues on the 6s. overall i recommend you get the lastest from apple so it will last you another 2 to 3 years or more.