Class 10 cards deliver on speed for HD video capture, which goes along with the size advantage of the SDXC spec. For more details on what all the classes and card specs mean, check out my resource article Demystifying SD Cards.
B&H and Adorama have a deal on the Lexar 400x Professional 128GB SDXC Class 10 UHS-I card for $119 right now (reg. $175). Check it out here at B&H Photo and here at Adorama. Free shipping at both retailers.
The Class 10 rating is a rating intended for evaluating video capture performance. In this case, Class 10 is the fastest rating currently available for video performance, which means these will work with just about every HD video camera on the market today. For a more detailed explanation of SD card classes and speed ratings, check out my prior article on Demystifying SD Cards.