For listening with my eyes closed I would probably keep the NAD. For listening with my eyes open - well, that's another story. For me, NAD doesn't have enough on Marantz in the sound department to compensate for what it misses in the looks.
Another problem is that you know the NAD and I assume it works fine. When you buy vintage, Marantz included, there's often issues to address, sometimes it takes weeks to fix the bugs. Out of 12 Marantzes from the 70s that I've bought over the last 5 years, half went straight to tech, in 3 cases repairs took few weeks, in 2 cases I had to get another tech because the first one couldn't get rid of all bugs. They are always fixable, but it takes time. Just bear in mind that if the NAD is your only amp and you sell it first, you might end up with no amp for a while.