There are a lot of differences between the two SSDs.
The main difference is that the 520's are for use in laptops and desktops.
The 710's are server grade which are designed to be used in servers.
It really depends on what you want the SSD for. If it is for a Desktop/Laptop with a SATA III interface then the 520 is your best bet. Compare the specs of the two SSDs.
Another SSD to consider for Desktop/Laptop use is the 320 if your Desktop/Laptop has a SATA II interface.
Hope this helps
The 710 and 520 are very different SSDs. The 710 was introduced before the 520, in Q3-Q4 2011, the 520 Q1 2012. They use different controllers and NAND storage chips, and the 520 is SATA III capable, the 710 SATA II. The 520 is a consumer oriented SSD, the 710 an enterprise (business) SSD. Finally, the 710's price is over three times that of the 520, for somewhat similar capacities.