Well the Marantz 6006 has an almost identical color GUI that the denon's have, and the onkyo 709 is a great receiver, but I'd never get a model lower than 8xx level from onkyo and even then, I'd probably just wait until I could step up to the 100x, 300x, or 500x.
I can say the Marantz 5006 is an awesome receiver and that on screen GUI doesn't have to be a drawback if you are connecting the receiver to a router. If you're marantz receiver is network connected you can control nearly everything, and I mean nearly everything from the web GUI, which is easy to use and fairly intuitive.
That has to be the biggest gripe with my onkyo, that the web GUI is beyond useless. However I would say sound wise, the marantz and the onkyo I have sound similar, with the edge going to the onk, but that isn't necessarily because onkyo is better. The onkyo I have is a 3009 and the marantz is a 5006. So they are in different leagues.