Every phone will probably release a new model before your contract is over..The thing is with apple products, they seem to release a new model every 6~ months, which might seem like a good business move, but I might prefer an Android phone because I get an iPhone 6S, it'll be inevitable that there will be one coming extremely soon.
And unless you want a slightly thinner, slightly faster, slightly better phone, with the 7, sticking with the 6S is fine. I mean, you'll still get all the iOS updates, and it'll still work fine.. I mean, my iPhone 5S is only slightly heavier, slightly smaller, slightly slower, and not sensitive to touch, and that's compared to two models on..
And compared to android phones, with all the different brands of phones ect, you're gonna get a new phone coming out every month or two, so like..