Frack... Okay... This is hard.  
Nolan is a master craftsman, his visual power is on point and there are all these other factors as well that make him great: casting, emphasis on strong and intelligent plots with strong twists, unafraid of a certain amount of open ended interpretation (he also does a ton of OVER explaining too but, cest la vie...) and he creates detailed worlds. But...  The Coens "voice" is so much more singular and relatable at the same time.  I do find Nolan's films a bit on the cool side, usually with one or two actors in them that manage to really break through and feel vividly fleshed out and not simply chess pieces for the plot or mouth prices for a certain point of view or emotion ONLY (someone TALKING about the emotion of love doesn't automatically make them emotive, especially if it's just to further some thesis the film has going),as much as living, breathing human beings.... Sorry, that's how I feel.  
The Coens also are masters of their craft but there is more vitality in the other areas, especially the characters, no matter how ludicrous some are (though yeah... They have gone over board with the ludicrousness at times).  They have produced a richer pallete of characters and stories I think as well.  And call this unfair if you will but... I don't think Nolan has a comedy in him.  The Coens can do RAISING ARIZONA and pivot off to MILLER'S CROSSING and both films will reach the highest levels of quality in their genres.  
As good as Nolan has been, and he's had a hell of a run... I gotta go COEN BROS.