Whenever a new phone / iPhone model comes out, it is usually better. But if you have the iPhone 5 model and it works fine for you, then upgrading is just a personal preference. In general, iPhone 6 has better display; screen is great; has faster speed; has a better camera; the screen size is bigger, so you might want to test it in your pocket first before deciding. Here is a great comparison reading:
Unless you need faster speed and better camera and bigger / better screen, then it comes down to your preference. Here is even more details comparison:
Please let me know. Thank you.