Would it make any difference?
Well 5D2 takes replacement screens easily and has exposure adjustments built-in. Not sure about the 6D, can't be worse than the 5D3 for MF.
Liveview is the best way to MF, if you can.
My 5D2 has the precision matte screen which really helps with MF lenses and focus confirmation and DOF preview.
This is a critical issue for me, and one of the main reasons why I'm not interested in 'upgrading' my 5DMkII to a 5DMkIII. I also do not consider the 6D an upgrade to the MkII; the controls and build are definitely not of the same standard even though it does accept different focus screens.
So my suggestion is to use a 5D MkII !!