I have used SanDisc cards since I switched from film to digital cameras. I shoot a 1Dx and a MKIV.....what is the absolute best card that I can put in my cameras?
Be careful on the 1D MK IV, SD speeds are indeed hampered by the controller (in the camera). So doesn't matter what card you put in beyond a 20MB/s card, you'll find no difference. Which means if you are writing to both then the SD is a "bottleneck"
Both the MK IV and the 1DX can do UDMA 7 in CF so the latest cards will give you the best performance. Of course this is more in terms of how quick the camera flushes to the memory card. I recently tried a 60MB/s CF vs 30MB/s on the MK IV and was pleasantly surprised by the difference
Finally, you perhaps might want to consider trading off size of memory card vs speed. By this I mean I'm not happy with more than 8GB card for normal (16GB for sports nature) on the basis of how much data I am prepared to lose should a card fail. I've only had 1 CF fail - a transcend, which failed before I used it in anger (failed during soak testing)
Best therefore does depend, as mentioned, on your needs...