This is a good question. If you have the tradition of getting the ’s’ model phones then it would suggest that it might be better to wait another year around September 12th 2015 for the next iteration.
There is some argument to be had that the iPhone 6 is a major departure from the 4 models. Bigger screen being the major factor.
The now classic iPhone 5S models. image: apple.com
Ultimately it depends a bit on what you use your phone for anyway. If you use your phone for watching video then this might be worthwhile upgrading. But if you are just using your phone for occasional messaging/facebook etc then it is not worth upgrading and waiting until the inevitable iPhone 6S Plus.
*This all assumes you are in the unlocked market too.