You are really stepping into a different world going from 10x to 15x. Many people can hand hold 10x just fine, very few can even think about hand holding a 15x. There is simply too much image shake without using the 15x from a tripod. That being said, I agree that the Kaibab is a much better binocular than the Viper.
You need to be sure you need the extra magnification. You don't give any clue as to your use, but all too many people think "upgrade" means "more" magnification. If you don't need the magnification, then you probably won't like any 15x, unless of course you are scanning from a tripod.