I have an S6 Edge. When I got it on end of term of my old contract, I had the one with the BTU firmware. It's unlocked and unbranded. Everywhere you go people will say it's better to have that than carries branded as it will get upgrades first. Guess what?? EE rolled out the update before I got mine. It may have been due to the beta launch but I was certainly at the back of the queue. Would I bother? MM isn't especially exciting and I discovered battery life got worse although I found an App called Doze on playstore and battery life is now better than Lollipop. Flashing is easy to do. If you mess up Samsung Smart Switch offers recovery options and you will need it to back up your device.
If you decide to give it a go and get any problems, come back and we'll help you out