Go with the 128GB. Apple stopped selling the 64GB in its own stores. If you per chance find a 64GB it would/could be harder for you to resell it after a year (or sooner).
Depends how long you'll keep it for and what you'll use it for. If you store a lot of movies, pics and videos go with 128. Nothing wrong with 64. It's got plenty of storage for some users.
Particularly if you're going to keep the phone. Better to have more than you need than less. It's easier to use it more than it it is to use it less But gone are the days where the 16gb base model was less than enough. 64gb base model is definitely more than enough for some (I have a 64gb and it's fine), so if you're going to have to stretch to pay for the 128gb, you should be fine with the 64.
I got a 128GB. It was about five to six bucks less than I was paying for my Note 7 per month.
I guess it also depends on how you're going to pay for it, like a device payment plan or outright or, if your carrier still has them, a two year contract.