M.2 SSD or HDD?

[Statik]

Hey guys, so I've dropped more money than I would like to admit today on my new UW 1440p build. The last thing I need is storage. I'm taking the two 840 EVOs (x1 250gb and x1 500gb) out of my current build. I don't need a lot of storage on my PC (right now my 250gb is 3/4 full, 500gb is about 80% full of games, and my 1TB HDD is like less than 20% full).

 

Since I've spent so much I'm trying to save as much as possible on this, and that's why unfortunatly I think Gen4 M.2 is out of the question. But my question is would I notice much difference if I bought a M.2 SSD for my boot drive (a 500gb or maybe 1TB), over just buying a 1TB HDD, and keeping my 250GB SATA as my boot drive. If so, which SSD do you recommend? Thanks!

[SavageNeo]

The a-data sx8200 pro 1tb m.2 gen3 ssd is pretty good for the price like 150bucks or so.