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When I shot Nikon the 1000X Lexars were almost as fast as the XQD.
In the 1DX it performs very well. Also paired with the USB3 card reader they make the download times to the computer are reduced significantly.
I was wondering, as you do now, whether or not save a few bucks and buy 800X cards. I am glad I decided to get the 1000X, because the buffer of the 1DX is not as big as the one in the D4 and I wanted to make sure I could keep on shooting as fast as possible and not having to wait for the buffer to clear in order to keep on shooting.
You have a great camera that was not cheap, get the fastest card you can afford, unless you will never shoot @12fps., but that is one reason we get the X.
I found that the 32GB 1000X cards offered a good value. They are at a good price over at B&H, they have the 2-pac for $239. The 64 and 128GB versions cost more per GB, but I like not to pout many eggs in one basket, hence I like using the 32's.
You have a fantastic camera that was not cheap, get the fastest card you can get, unless you shoot only single frames and never continuous 12fps, but that's one of the reason people get the X.