With a 512GB SSD in my 2013 MacBook Pro, I only had about 180GB of free space available, which regularly overflowed on my photographic outings (I shoot raw only, not JPEG).
When I purchased the 2013 MacBook Pro, I was pretty sure that 512GB would be enough, but 18 months later that assumption proved wrong.
So this go-round, I considered buying the 2016 MacBook Pro with a 1TB SSD, which will be enough for... how long? And the SSD in the 2016 MBP cannot be replaced, being soldered onto the logic board.
Then there is the issue of longevity: a 2TB SSD used to significantly less than its capacity (say 1.5T or less) has in effect has far more “over provisioning”, so that when blocks of flash fail as they inevitably do, there are ample unused blocks around. Compare filling a 1TB SSD with 950GB of data versus filling a 2TB SSD with that same 950GB; there is no “wiggle room” on the 1TB SSD in either storage capacity, or in unused blocks to deal with flash block failure.
Thus my decision to go with a 2TB SSD. Now I have ample space to take along whatever I need, as well as ample storage for my 7-14 day shoots.