I had to settle on the 16GB as it's the only 6+ that showed available on Apple's site last night, the 64GB was showing 7-10 days shipping and I didn't want to wait. I think it'll be fine, all my iPhones until now have been 16GB and it hasn't been an issue.
You couldn't wait? There's a reason why 16GB was available because no one wants it.
iOS 8 apparently needed 5.8GB just to download over the air, good luck.