Get what you can afford. If your getting a phone on a contract, I'd go for a 6S simply because you'll probably pay less per month than the 7 will probably end up costing at launch. Also there are no officially confirmed details for the 7 yet so you may end up using the 5C for longer until the 7 is announced and then decide to get the 6S anyway. (Although you might get a better contract price on the 6S by waiting until after the 7 launch).
Apple do support their products well so you'll still get plenty of updates for the 6S if you decide to go for that.