I can relate to this story...
I was a proud owner of an '11 E92 M3, with all the bells and whistles, DTC, competition pack, EDC...my wife was driving an '11 F25 X3 x30d, again sport pack, all the whistles and bells...the M3 with its mighty V8 going to 8000rpm was a wet dream...I loved that car...but then, our new socialist government decided that they need even more money and beginning of '12 a new tax on corporate cars was introduced based on CO2 and list price with all options...this meant that both the cars would have cost me an additional 20K€ EVERY year of ownership
...I went through a painfull and long process of what to do...sell both cars, which were the best ones I (and the wife) ever had and buy something which will go under the new tax radar or to just bite on my lips and shell out the dough every year...
Eventually, after a lot of number crunching and thinking about replacement cars I bought my wife an F30 320d sportline (well equipped)...car is not in the same league as de X3, but drives very well and is dirt cheap in taxes...next thing was to replace de M3...I was briefly thinking about the F30 330d, but then I would get taxed again
...the 520d, I had my reservations as I always had high power cars...I then did test a demo 525d (2.0d TT), very well specced and was very pleasently surprised...no, it doesn't even come close to the M3 experience, but it has plenty of power for daily use, comfort is x10, and if I now look at the running costs of both cars and compare to the X3 + M3 ... it's just rediculous...
Bottom line is that I went from a very special and powerfull (for me) dream car to a sensible DD diesel and I'm not looking back...yes, when I'm in Germany, on an empty autobahn, I wish I still had the M3, but that is 0.5% of the time...Like someone said, one was bought with my heart (passion) this one was bought with my head...in your case it might be more difficult, but our government made the decision much easier for me...I HAD to listen to the head...
So far no regrets at all, but I do think that the 525d, has this extra little edge over the 520d, which gave me a bit more peace in my head to make the switch...