I've posted a video that shows the difference between booting and starting Firefox from a "green" drive and an SSD here
. Beyond that, it's really an individual decision. For me, it's a no brainer, spend the extra money. But if money is dearer to you then you might arrive at a different conclusion.
I will opine that a "green" (~5400rpm) drive is a poor choice as an OS drive both for performance reasons and because it spins down after a period of inactivity. Since the OS drive is frequently accessed by the OS, the result is that the drive often spins up and down repeatedly.
An SSD for the OS and either a standard or green drive for bulk data storage makes a lot of sense, IMHO.