I had the 5s before upgrading to the 6.
The 6 had three features I was interested in:
1) Thumbprint
2) Calling Over Wifi (Where I live, I get HORRIBLE reception with T-Mobile. Wifi calling fixed that)
3) Larger Screen
There were some other enhancements like faster processor but the above 3 were the main things I was interested in.
But the best part was I called T-Mobile and they offered a trade in program. I'm not sure why but they offered me $50 more than what the 6 was going to cost so after I traded mine in, they sent me a Credit Card for $50. Works for me!
I believe the 6 has a better camera and video section than the 5s also, I doubt you were going for that, being the camera guru
I got my daughter a 6 yesterday, seems very nice. I have a 5s, the 6 is a nice step up in screen size without being too large like some phones are. The 5s has a fingerprint reader.