Apple just released the first update to iOS 8, and it looks to be taking down iPhones. The iOS 8.0.1 update is rendering TouchID useless, and crippling cellular reception on iPhone 6 models.
That being said, you may want to hold off on installing the update until Apple fixes the bugs which are being reported. If you haven’t installed iOS 8 on your old iPhone, you may want to wait on that move as well.
The iOS 8.0.1 update packs Apple’s Heath Kit software, and was supposed to fix some bugs also.