I sure as Hell don't need 64GB on the device. On the other end, I don't think I need a data plan for more than 2GB/month based upon my figuring 1 page of data is ~ 60kb (off the top of my head). I don't think I'd do more than 10 pages of anything in a month. Talking, 1 and once in a while texting would be the limit.
I was looking at the last year's 6 because I want to use the ApplePay.
Am I right? Wrong? Half and Half?
It's called iCloud now and yes, You can share between all your Apple devices. Photos can be shared. My girlfriend and I just spent four days in Seattle with both of us taking pictures. I set up a shared folder in the iCloud of my Seattle photos and then my girlfriend simple added hers to it. Now we both have all the photos from Seattle available to us. Easy.
There’s no right answer. What works for you is right for you. I’m 64 and filled up my 128 GB iPhone 6 Plus in several months.