For owners of BOTH 1D series and 5D III bodies. Need your wisdom!

I have been shooting with 1D series bodies for years.  I am used to the feel, layout, and user interface such that muscle memory allows me to adjust ISO, focusing point, metering mode, exposure comp, ap/shutt spd, etc., without having to think or take my eye away from the VF.  Obviously, the 5D series has a different UI design such that it will take some time for me to make the same connection to this new body.  In the end, I think I am planning to leave the 1D line altogether given the majority of my shooting needs (which, most importantly, gives me the option to have a more compact body).

For those of you who have shot regularly with both body types, can you please tell me what you feel is the greatest challenge when making the adjustment to using a 5DIII?  Because 1D series bodies and I "have become one" I need to be psychologically prepared for this switch.