This in my opinion is a unique year. In previous years the entry level, mid-level and top-level has been different.
Last year: 16GB, 32GB, 64GB
This year: 16GB, 64GB, 128GB
To break this down even more for this year.
The mid-level is 4x the storage capacity of the entry level iPhone.
The top-level is 8x the storage capacity of the entry level iPhone.
In previous years it wasn't enticing to upgrade, but in my opinion, this year it is worth the price cost from entry/base to mid-level. If you like to store a lot of content on your phone then the 128GB might be the way to go. However, I feel the 64GB is the best "bang for your buck" this year. Whereas in previous years I've always recommended the 16GB. Only time will tell.