Personally I love HDR for the extra brightness in strong sunlight, reflections, and nighttime lights. But using HDR can complicate your workflow for working with screenshots and screen recordings, because they may require conversion before posting. If one gets into it, beware there are significant differences available in HDR implementation, in terms of maximum and minimum brightnessess, and the ability to precisely dial in the right brightness for different parts of the screen. Do your research before you buy if you go this way. 
As for G-Sync: yes yes yes! It lets your sim run at its natural frame rate without any tearing issues. Note that many monitors these days are “G-Sync Compatible” or something which is not 100% exactly the same as official branded G-Sync but usually is fine. I do recommend doing a little more research on this if you’re curious about the details, though it’s not as complex an area as HDR. 