Looking for feedback on battery and bluetooth

Hi all. I have recently got a small portable braille display which can be paired with my IPhone 6 via Bluetooth. I'm also considering getting an Apple watch, just waiting to hear if a new one is soon to be released. My question to people who have either of these devices is what can you tell me about the difference in battery useage on the IPhone with Bluetooth on and devices connected? I know that the apple watch needs to be constantly connected to the IPhone via Bluetooth, so how does this affect how quickly the battery runs down compared with the IPhone with Bluetooth off?
Any feedback much appreciated!