Putting games on it is different than storage. Get the fastest you can afford when ready to buy. As noted above, that can change depending on when buying happens. The answer now might not be the answer then.
Edit: Let me point out what I mean. I'm looking for a 1TB SSD for my system. I want to move my OS to my 840pro, and move my steam folder off my 840pro and onto a good 1TB drive. Usually that means I need to get the MX500. The 1TB MX500 is usually sub $200 and the cheapest quality drive up there. One of the last times I looked online I saw the MX500 was $180, while the Samsung 860EVO 1TB was $170. Sales and when you look can change the answer. (I just looked, both drives are currently $170 on Amazon.)