which grasslands is better

Between the two, I would recommend the former, where you can see better grassland and enjoy yourself by horse riding. As the latter is relatively closer to Hohhot, there are more tourists there, so you will find things there are more commercial.

To have fun there in such a limited time, you are recommended to rent a car to the grassland directly. The car rental is usually available at the main bus terminal and train station for your reference. If you have a guide to arrange that, that would be better. With his or her help, you will get a chance to stay with a local family.