The Regular 9" Box Spring
The Regular 9" box spring has the traditional height for a mattress box spring. If you want to have the same size box spring typical in mattress sets, then this one is right for you. However, if you're ordering a much taller mattress than you currently have, keep in mind your bed will be sitting higher. So if you're ordering a taller mattress, and would like to keep your total bed height from getting too high, you should consider the Low Profile box spring.
The Low Profile 5 1/4 " Box Spring
The Low Profile 5 1/4" box spring has become popular as the mattresses themselves have become much taller. In recent years, people have been choosing taller mattresses providing greater comfort and support. As mattress manufacturers responded to this demand for bigger mattresses, they also created the complimentary Low Profile box spring as a way to keep the total bed height reasonable. This makes it easier to get in and out of bed. Also know that the Low Profile box spring provides the exact same support and durability as the Regular 9" box spring, so your choice is only about the height.