14 DAY TRIAL // If you want to be boggled by a small memory card that has twice as much storage capacity as most solid state drives (or more), you'll want to check out the 512 GB SanDisk Extreme PRO SDXC UHS-I.
If it's performance you're looking for though (even though the 90 MB/s of the other one is more than enough for 4K recording), the SanDisk Extreme PRO CFast 2.0 memory card might be more your thing.
You see, not only does it have 128 GB of storage space (SSD caching drives, full-sized SSDs that is, have 64 GB – 100 GB), but it also works almost as well as the best solid state drives (SATA ones anyhow).
Specifically, the data transfer speed is of 515 MB/s, which is a lot. The best SSDs usually hover at 550 MB/s, and their controllers have a whole bunch of chips to work with at once.
As for the writing speed during photo/video shooting (when the card is used in a camera/camcorder), it is of 440 MB/s. You can definitely store a lot of 4K video (4096 x 2160 pixels resolution) on those 128 GB.
The SanDisk Extreme PRO CFast 2.0 card is a successor to the CFast 2.0 launched last year. It is made for professional broadcast, photography, and cinematography industries.