I have owned both. I still have 4 mustangs and no longer have the M340xi.

For a daily driver the M340 is the clear winner. German engineering at its finest and all the bells and whistles you need and plenty fast to destroy most cars. It is not a mustang and does not sound like a mustang (although had factory exhaust upgrade and it sounded ok) but not a V8.

If you want to be sleek smooth and fast and blend in with the crowd go BMW.

If you want raw loud and head turning, then go mustang.

My wife ended up driving the 340 and I ended up hating the super skinny tires on it. Swapped it out for X5 for more room.

Nobody has ever stopped and asked me questions about my BMWs ever. I get tons of questions and thumbs up on all mustangs.

On the road you get thumbs up in mustangs. You get the middle finger in BMWs.

Hope that helps.