Lets just put the cost and performance aside for a minute and ask a different question. How much does it matter to you if the car is a ///M or not? As cocky/douchey as it sounds, owning a M is a completely different experience than owning a regular 3 series.
Its not like the difference between a Civic DX vs Civic Si or even Civic Type R.
Think about it, a M twenty years later will still be a cool car that turns heads while a 3 series twenty years later is just another old car used to get groceries that's not as reliable as an accord.