The M3 may not be your ultimate dream car, but you have been pining for one for a long time. And I don't recall similar pining for the GTI.
So, I would suggest buying the the M3 will ultimately save you money. Because you will not spend the GTI money, plus the correction to get into the M3 you should have bought in the first place.
Plus, your wife has always pined for something a little more upscale/luxurious. She might even prefer the M5.
At least that is an outside observer's take...