I prefer the G series over the M series due to the handling and size, however, my wife would prefer an M due to the size and interior.....So, they are very different cars in my mind with a different target audience.
I'm not into leasing, have always been told financially that is the best option for the dealership, which would mean not the best for the customer. However, based on your situation it might serve you well.
The G37xS is a great car! If they are making deals on them, then I would pull the trigger as opposed to getting the Q50 since it sounds like money is an issue with you......but, since I didn't win the lottery this week it's also an issue with me!!!
The last few cars I've purchased have been slightly used cars still within the bumper to bumper warranty. Have you looked at this option??