I too am a long time BMW driver (actually, I had a 911 before any BMW, but for the last 18 years, BMW).
I haven't driven the new X3, but I can tell you that the Macan's ergonomics aren't as good as my M4.
The ride, handling, steering, seat and PDK are really good - these are areas where the Macan is likely to win over the BMW.
Where the BMW wins:
1. iDrive is faster to use and find things - Head Up Display is also very useful.
2. Although the surround view is good in the Macan - it's not as accurate as the BMW. The Porsche engineers have built in a tolerance - so you think that you are parking really close to the kerb, but really you are still 50mm away.
3. With the BMW you can get side view cameras in front of the front wheels - this is really handy for me, as I often need to pull out of a lane into traffic.
4. Voice control - It's actually useful to call someone. But apparently CarPlay can do that well. I have Android, so I can't confirm that.
5. Knowing what setting the chassis / ride height / sport mode etc. the buttons on the centre console look good, but in the sun, you can't see if a setting is on or off. Plus you need to look down.
6. Just finding settings is easier in the BMW.
7. Things like being able to switch off the engine, then wind up all the windows, close the sunroof and look at the driver's mirror for a cyclist / car - then open the door and, whilst opening the door press the mirror fold button.
All these little things really annoyed me for ages, when I first got the Macan, but now, I just deal with it.
Would I get a new Macan now, or a new X3 ? - I don't know, it depends on how well the X3 drives.