I'm thinking of buying a 12w apple adapter instead of using my 5w adapter for my iPhone 6plus.
And i would like to know if it only speeds up your charging time?
Has anyone verified that this doesn't have any negative long-term effects on the battery life?
Apple says so. You can use any charger that fits to charge the phone including the larger chargers for the iPad. I use one iPad charger for all of my devices.
You know it only charges the 6 & 6 + faster though?
If you've got any other model of iPhone, any 'faster' charging is purely placebo, as the phones are designed to top out at 5w (sorry if you already knew that!).
I've largely done away with Apple chargers and gone with these guys. They're not much more in cost than the Apple chargers, but can charge multiple devices at once, at the max amount of current each will safely take in.