I use a CF Card and write at the same time to the SD card, gives me two copies of everything I shoot (on the 5DMK III).
To cover the speed issue on the Canon Buffer/write issue, I use a 128GB CF Card @ 100Mbs + 64GB SD card @ 95Mbs, so slowest write speed should be the SD Card @ 95Mbs, which I can live with.
I do this as I have twice now had CF Cards corrupted, fortunately with Software I've rescued the Images, so now I cover my Bases by writing to both Cards, I do the same on my 1Dx, but both CF Cards are 128GB 100Mbs, so not an issue like the 5DMK III.
Try to find a situation where the write speed on your SD Card is as close as the Write speed on your CF Card, then the issue is no longer an issue and you have the convenience of a set of "Back Up" Images.
You will never see close to a 95 MB/sec SD card write speed on a 5D MK III. You can put a 95 MB/sec card in a 5D MK III, but you will eventually get about 10 mb/sec due to the type of SD controller in the camera.
The card speed is limited by the camera.
Speed advertisements for SD cards are misleading, since they only apply to a new and blank card, and they are for Cameras with UHS-1 controllers which have a internal bus speed and thus write about 2X faster.
Initially, you might get 20 MB/sec or even more in your 5D MK III, but ... once you have filled up that SD card, and do a in camera normal format, the write speed drops to 10 MB/sec or less. This is because the controller must first go thru the slow process of erasing a memory block before writing to it To regain the lost speed, you must do a low level format which erases the card. It can take hours to do that for a large card.
All the reviews for SD cards are for a card that has had a low level format first.
Quoted for truth....
SD cards are THE worst option for doing photography with the 5D.
I did a write test on a transcend 16gb class card (one which I normally use to shoot with my Sony FS100, and was getting around 11 mb/s in the SD slot. That is f****** useless and a joke that it's even there.
Don't save... just get the CF card. If you use an SD card at the same time, the SD card slot will make the CF card slower. Lowest denominator rules in Canon apparently. What a waste.