Bottom line: I wouldn't store a big photo library on a single mechanical HDD that's mounted within a portable computer. So you must have an external drive regardless to fit them all, and here's the question - do you still need your entire library on your internal drive? Do you need a big part of it on your internal drive?
This was exactly my rationale for spending $$ on 16GB RAM and keep the SSD at 256GB. I had thought to bump to 750GB but just too pricey.
But, even at 512GB, I would still run out of space, just not as soon as with 256GB. EXTERNAL storage/archive would have been necessary in either case.
My only concern as I migrate my work stuff (large client DBs and other data files), and then my photos, is segregating archive (external) from active (SSD), and then remembering what is where... This is a slight concern - we shall see how things progress.