Depends.
Best answer so far.
Depends.
I would if you really like the screen size and design. The 6will be thinner, and have a larger footprint so if you like the current size, go for it. it will get software updates up to must likely IOS 11, so its not like it's obsolete.
Apple aren't stupid, they're in this business to make money. No way will they allow the 5S to run great for much more than another 12-18 months.
Well im assuming the OP means on a 2 year contract, and its nlt like you are required to update your ios version every time a new one is released