I think this an apples and pears thing - there is not a lot of point in planting your flag in one or either of Cee Lo's or John Legend's camp, as if they were competing over the same ground. What seems to have started it is the reference to Cee Lo as retro-soul, but I think that is a bit misleading. For sure, he references soul, as he does pop and funk but to me in a completely different way to JL. Cee Lo is closer to Outkast than someone like JL. It is a very knowing and playful take on black and white pop music, and no surprise that he got on famously with Danger Mouse who has a similar love of all forms of music, as well as a very modern way of re-engineering it - much closer to the patchwork sampling of hiphop than classic forms of songwriting.
One thing I would say is go and see Cee Lo if you get the chance - there is whole other dimension (literally) to him that you just don't get on record. Larger than life just doesn't do it justice. This is a demented kind of opera, in the best possible way of course, a voice which could take the roof off, and a man who absolutely loves to let it rip.
While I absolutely respect what John Legend has done, it is completely different. Cee Lo for me is joyful and inspiring and funny, but in no way a retro act.
So let's not set them up as competitors for the same ground.