It all depends on an individuals needs and the way they do their computing.
The SSD only dictates the onboard storage. When taking external storage into consideration you have potentially unlimited storage. Ask yourself how much you need on your laptop at all times. Look at how external storage affects the convenience of use.
I like to catergorize my data as many do. Operating software should always be on the internal SSD. Stored data used by the operating software can easily be put on external storage, but is less convenient. Obviously we would all prefer the highest capacity SSD available, it comes down to money and priority.
I just got my rMBP 13" base model yesterday, 128GB SSD. I have a Transcend Jetdrive 128GB on the way. Just prior I got a portable external 1TB USB3 HDD. This situation will work for me. Perhaps it would work for you?
Gather information, evaluate your situation, determine your priorities and you will come to the best course of action for you.