That was a dumb response.
Depends on your priorities. People complain about build quality because they expect perfection from a car of this caliber. The Mustang will be lower quality everything all around. M3 will be expensive to repair if something major breaks down the road. If you can't afford a few grand for a repair then you should not buy an out of warranty BMW. Heck, the windshield on my 545i was a thousand to replace.
Buy what you love so long as you can afford to own it.