First, I don't have a 1DX. But I have used various focus screens.
For long telephoto work (135mm and up), microdiaprism such as the Ec-A is what's normally wanted.
For shorter lenses, under 135mm, a split image finder such as the Ec-B can be used and often works better than the microdiaprism. This type often won't work well with longer telephotos.... the split image finder "blacks out", at worse... or is just very hard to use.
A lot of older, manual focus cameras came with a focus screen with three different focus assist features: a split-image "dot" at the center, a microdiaprism "donut" surrounding it, and an overall matte screen for general purpose.
I do not know if it's the case with the 1DX, but on many other Canon cameras, using either microdiaprism or split image type, manual focus assist screens will significantly skew the camera's Spot Metering, for all practical purposes making it unusable. Perhaps the 1DX has a means of correcting for this (such as a Custom Function setting). You should check into this, just to be sure.
Canon also has offered the Ec-S focus screen, to help with manual focusing. That's a finer grade, brighter screen all over, without any microdiaprisms or split image rangefinder. It's sort of like the matte area of old manual focus cameras. This type is usable in most Canon cameras without any problem, with no skewing of the Spot Metering function.
You might find more info here:
http://www.learn.usa.canon.com/resources/articles/2013/eos1dx_interchangeable_focus_screens.shtmlI use the "D" or "grid type" focus screens in several Canon models. This doesn't help with focus, but does provide a "rule of thirds" grid that I think helps me keep horizons level, walls vertical and just makes for better compositions.