Two things:
1. Mophie is the best choice, always. They don't have one out for the 6/Plus yet so my suggestion is to wait until Mophie releases their iPhone 6 version, and get that. If you want to get a cheaper one or one that is already available, then feel free, however Mophie is the best one out there.
2. In my personal opinion, in case you're an extremely heavy user without a charging source nearby, I'd recommend seeing how your battery life is without a battery case and then get one if you need it. After the recent iOS updates, my battery life on my 6 has been nothing short of outstanding. I'm usually at about 50% battery remaining at the end of the day. I don't even have to be OCD about my battery anymore because it's been that good. It's much better than when I first got it, recent iOS updates must have made some tweaks to improve battery life. If you find that you still need juice before the end of the day after test driving it for a couple weeks, then go ahead and get a case.