Confusion on Selection Of SD card For My Lumia 720

The first one is a class 6, the other one is class 10.
The read write of the class ten is higher, so that is the better card, whether you would notice the difference, i can not tell you.
 
But both have stated class 10 on them.

Dietrich Cleijne is right though. Both cards are specified as Class 10 UHS1 cards, but read/write speeds differ. You should always go for the higher value, as it definitely provides lower access times, for example when copying/moving files, starting games, playing videos etc.
 
Go For Class 10, since Installing on SD Card is now Supported. The Faster the Transfer Speed, the better.