Pair of 3.5-Dujmovic: one for distributions, one for backups and movie — to expand the disk subsystem of the laptop.
A pair of 2.5-Dujmovic mostly lie idle, are occasionally used to transport large (does not fit on 16GB a couple flushes) doses of information.
Discs mostly left over from older configurations of computers. Just buy more boxes to them. Only one 2.5 bought. If there was one to take such a disk to borrow or rent, would not buy, because it is rarely needed.