You'll gain significantly faster boot times and just general snappier performance on the OS itself.
120GB is more than fine for the OS and some programs. Use a separate storage drive for everything else. (I strongly recommend the Western Digital Black drives).
Overclocking will net you faster in-app performance, so slightly more frames in games, some time shaved off video rendering etc...
It really comes down to what you want out of your system, I would say the SSD is the more logical choice for its speed compared to a standard mechanical hard disk. Providing you have a decent enough board and CPU right now, you probably won't gain as much of a noticeable difference, at least, not as much as an SSD.
I haven't personally got the appeal to the Z97 boards either. I mean, they're great, as are the chips that go in them, but the next Intel CPU iteration is so close on the horizon that it just seems a bit pointless to do an upgrade to it now. It makes some degree of sense for a brand new build.