Size is more important that speed, until you hit 16GB, then speed is more important than adding more.
With 8GB, you will be waiting a long time for assets to load in modern games, and you will frequently see stutters when new assets are loaded into RAM mid-game
With 16GB, you will load all of the assets the game needs in the loading screen, and there would not be any need to load assets mid-game. This is because games are optimised for 16GB right now, since that is what the major consoles have. It will be a very long time before 32GB becomes the optimisation target, maybe 5 years or more, once the PS6 is out.
However, 1333Mhz is pretty slow for a modern system. Assuming you have 16GB is both cases, then the difference in games between 1333Mhz and 1866Mhz is about 20% framerate on an AMD system, so can be the difference between 45-50FPS and rock-solid 60FPS locked to the refresh rate of your monitor:
The difference in price is not large, so I would definitely recommend getting 2x8GB 1866Mhz or 2133Mhz if you have an AMD CPU.
If you have Intel then 1333Mhz would be fine.