It's better looking than the Mazda (imo), great to drive and much better residuals than the Mazda. Reliability and build quality is also excellent, though I haven't checked Mazda recently.

Just thought I'd pitch in cause my mate has a BMW 1 series and I have a mazda 3 (so can't talk for the 6 but I think 1 series to 3 is a better comparison anyway).

Both our cars have similarly sized engines and power outputs, my mazda is marginally quicker in acceleration but neither are exactly quick cars. I personally think my mazda is more fun to drive, the steering has more feel and feels like it has a quicker steering rack to me, though honestly both are great in terms of being fairly fun to drive compared to similar sized hatchbacks.

I reckon my mazda uses more fuel, we both drive in very similar ways as well so it's a fair comparison.

The build quality, especially the interior, feels solid on both. Honestly no winner there. I prefer the styling of the BMW both interior and exterior, though if I'm honest I don't like some of the internal plastics on the BMW but that's entirely personal, objectively there is nothing at all wrong with them. The seats in the BMW are far more comfy, both back and front.

The BMW is quieter on the motorway at speed, without a doubt. But honestly the amount of buffeting on his BMW when the windows were down was ridiculous. It made my 20 year old peugeot look refined! Easily solved by moving the window slightly up or slightly down though to be fair.

If was OP and I had that much money to spend, I'd personally not buy either of these cars because you could get something much much better second hand.

But neither are a bad shout, though if you really wanted new I'd probably get the BMW on lease (some insanely good deals on them at the minute).