Keep / get the donut.
For additional $150 it is well worth it. (I custom ordered my car with it)
The entire thing is removable and you will have the same trunk space as a car with RFTs. The trunk floor is slightly different but you can order that part separately and have the perfect OEM look.
My M340 had a nail in the tire it within 1st 700 miles. Eventhough I did not have to use the spare, it was a piece of mind.
I can write down pages and pages of nightmare stories about being stuck on the side of the road with no spare tire. Those quick fix things are useless and will immediately destroy your tire if you use it.
To me, it is an absolute no brainer to get the non run flats (heck we have been bitching about RFTs for the longest time and BMW listened and offering us options now) and get a car equipped with a spare.
If you were to buy everything that comes with the spare tire package, it will cost you well over $150 and I love my regular tires!
I will not leave my house on a road trip without a spare, period.