They both look like 75 Hz, 1080p, TN monitors. However the Asus is 3 inches larger at 24". Free-sync doesn't work with Nvidia cards, but it doesn't cause any problems. In fact, if the Free-sync works over HDMI, you can use it for an Xbone, and possibly the PS4, if you have one.
Personally, I find 21" monitors to be too small. My first 24" was an amazing difference and the 27" I got 18 months ago is another nice improvement. I also wouldn't buy a TN panel if I had any other choice. But it all comes down to what you value. Monitors are items I still prefer to see in person and buy from a brick and mortar store.
What's your budget and what do you want from your monitor?
You can use a Freesync monitor with your Nvidia card. Being 75Hz that's a good speed for the monitor as your GPU should hit 75FPS at 1080p in most games with some quality details and AA turned down some. However the refresh rate won't match drops in FPS performance like they would with a Gsync monitor (or AMD video card). What you can do is create a custom monitor speed to match your card's performance like say locking in at 60Hz to guarantee a smooth stutter-free experience during heavy scenery that dips FPS.
I agree with the above on 21.5" size. Too small by today's standards. You'd regret purchasing it real quick. Also, if you want height and tilt adjustability, you'll need to step up in price like ASUS's 24" 75Hz Freesync VG245H.