I had a 2010 BMW 335i with the BMW Performance Power Kit and Exhaust and my dad currently has a 2011 M3. The 335i was fast in a straight line and I loved that car. But after driving the M for this first time I never saw the 335i the same way. It's great car no doubt about that, but it doesn't compare to the M3, there is just something about listening to the sound of that engine, plus the downshifts with the DCT. There is just a different feeling when driving the M.
For anybody to say that the 335i is better the the M3, they are just trying to reassure themselves that their car is great.
That's my .02