XB270HU is better not only due to GSync but also due to the better panel type (IPS).
If you put the panel type topic aside, I have XB270HU since April 2015...here is my experience...I hope you are already familiar that in order to use GSync, you need nVidia GPU.
These are my opinions beside that GSync will eliminate any kinds of screen tearing.
If your PC can push fps really high let say over 90 fps as example, GSync is a pure huge waste of money.
If your PC can only push moderate fps e.g. 30-60 fps, GSync will make the game somehow felt smoother. This is where I like having GSync. 40 fps game with GSync is still nowhere near the smoothness of 90 fps, GSync is not magic, do not forget this.
Does it worth the money, only you can answer.
Going from XG270HU to XB270HU cost more but it is not only due to GSync. The panel type of XB270HU is also better.