Aside from the speed difference (substantial) and the price difference (way less than it used to be), SSDs are much less susceptible to accidental damage. If you drop a running laptop with a magnetic disk drive on a concrete floor from table height, you will have an expensive paperweight. Your data will be current only as of your last external backup. If you drop a laptop with a solid-state drive the same way, you will probably destroy the screen and maybe mess up some of the innards, but the SSD itself will almost certainly survive. You (or a techie) will be able to remove it and recover everything that was on it. A dual-drive strategy will not allow you to do this, as your SSD will probably only hold things you could reinstall anyhow.

And do you know what sentence is said most often just before someone drops a computer? it's "Oh, I never drop my computer."