I recently just went through this exact same exercise between these same two integrateds and in my case I went with the McIntosh. Anyone who suggests that one smokes the other really has not properly auditioned both of these fine components.
There is much to like about the Luxman, it is beautifully finished, has a terrific onboard phono section and an extremely wide soundstage with great imaging and well controlled dynamics. I dont see how anyone could buy one and be unhappy with the decision.
Why did I buy the McIntosh instead ? You can listen to a number of upper end integrated amplifiers and there is one thing that the McIntosh brings to the table that is unique to the brand. That is the remarkable depth of the soundstage. The Luxman in contrast has an incredibly wide soundstage that can literally make your speakers disappear and sound much larger than they are all at the same time. However the McIntosh has this incredible depth that can literally hang instruments out in front of you in a way that is almost eerie. The soundstage may not be the widest, but it is certainly the deepest and I found that added a new very enjoyably dynamic to my listening. However depending on your room and the type of listening you do, this may or may not work for you.
The other aspects that appealed to me is that you can add a tuner into the 6600 (not a valuable feature for everyone but valuable to me) and you can be guaranteed strong resale value should you decide to go in another direction. I have already come across used 507us for significantly less then you will find a used MA6600. I know because I wouldnt mind picking up a used 507u to use in another system as I do think very highly of the Luxman.
They are both very good products and I cannot see being unhappy with either of them. Best of luck.