1MVO,
I think you are in denial over this decision you have made. You are getting up there in years and need to face the fact that an automatic transmission fits the bill. I'm totally kidding here!!!
Both cars are awesome and I think the E90 M3 and it's engine will be the last of it's kind. DCT, although not a manual is still badass. I just hope you had the buyer of the 1M sign a contract that he won't allow any friends drive the car.
Good writeup in your OP. The fact you stayed with BMW after considering other brands says a lot. The new 991 is just a mini Panamera and has lost it's edge. Until the 991 GT3 (better have a manual option!) the 911 is just a pseudo luxury coupe.