Not sure the real main difference between the 12 & 13. Probably on the video side of things. The 13 series can probably pass through the latest HDR, Dolby Vision on the 4k video side of things.
If you like the features, functionality and setup configs of the Marantz, I would highly recommend the new flagship Marantz SR8012 AV receiver. Looks like a good contender and is probably one of the best sounding Japanese AV receiver.
What are your speakers?