Using SSD as a boot drive, what else will you put on there? Depends on how big an SSD you can afford. But if it's just a $100 128GB drive, you won't fit much on there. Windows, maybe Office and a few other apps. Then what's your benefit? You can boot Windows really fast, and open those apps fast. Everything that is saved on the HDD will load really slow.
With a cache, everything is cached. All the programs you use the most. So if you open the same 4-5 apps every day (browser, twitter, messenger, photoshop, excel, whatever games you are playing at any given time from your 300GB worth of installs) they will load like they are on an SSD.
Or you could do what I do, have a 256GB SSD as your main Windows and gaming HDD, and have a normal HDD + cache it with another SSD
