Which version should I get 1.6.0 or 1.5.2 beta? And whats the difference?
1.5.2 has been released already and is no longer beta.
If you're using the Steam version, then you will automatically receive 1.5.2 (if you already have 1.6.0 you will get downgraded).
As for "which you should be using":
As long as you keep in mind that selecting tiles for event graphics will crash the editor and that moving maps around will mess up their order (and cause some of them to disappear), and if you're not afraid of messing with the .json files to work around these bugs, then 1.6.0 is not a bad choice.
Otherwise, 1.5.2 should be stable for most.
There isn't much difference between the two.
1.5.2 is pretty much 1.5.1 with Gene support.
1.6.0 seems to have better support for High DPI screens, fixes a couple of (relatively minor) 1.5.1 bugs such as context menus rendering outside the screen if your mouse is too far low, more stable save/load ingame. It also includes updates for some of the js libraries RMMV is using, which may fix some other bugs and/or improve performance.
Feel free to correct me if I'm wrong.