Also available is the Lexar 256GB Professional UHS-II SDXC card for $99.99 (reg. $149.99). Check it out here at B&H Photo.
New info from a “reliable source” at Canon Rumors points to the upcoming Canon 5D Mark IV and its change over to a CFast 2.0 card slot for its primary storage. The rumors note that it would feature both a CFast 2.0 card slot and a SDXC UHS-II card slot – both of which are a substantial upgrade from the 5D Mark III.
The UHS-II bus interface offers the ability to build cards with up to 312MB/s transfer speeds, which is a big jump from the maximum theoretical speeds on the UHS-I interface at 104MB/s. Of course, the UHS-II card slot will still be compatible with older UHS-I and non-UHS bus cards. You just won’t get the speed benefits that UHS-II cards offer.
Of course, CFast 2.0 offers even faster speeds with its SATA-3 connection at 6Gbps (up to 600MB/s) max data throughput. That is the fastest current mainstream memory card (XQD’s PCIe-based theoretical limit is 5Gbps).
Either of the card slots should be more than competent for whatever kind of compressed 4K video the Canon 5D Mark IV throws at them. The biggest question remaining to the bottleneck is Canon’s processing capability for still images. Is Canon simply going to build a 1D X Mark II Jr. as the 5D Mark IV?
B&H has the SanDisk 128GB Ultra UHS-I SDXC card for $44.54. As a UHS-I Class 10 card, this will work well for HD video capture and that high storage capacity will store a ton of compressed HD footage that most camcorders and DSLRs capture. Check it out here at B&H Photo.
You can learn more about SD cards and what the specifications mean in Demystifying SD Cards.