I'd prefer the Diesel due to it's motor characteristics and higher MPG. The Diesel price in Germany was traditionally lower than for petrol and will hopefully get back to this level.
However, there are serious reasons against a Diesel. The DPF (Diesel Particle Filter) is a source of problems if you don't drive longer distances regularly. The problem is, that there is an exhaust gas recycling system built in due to environmental regulations. The "recycled" exhaust gas will more and more clog the DPF which is cyclically cleaned at around 600 °C. This cleaning (kind of a burn-free) is triggered by an electronic controller, preferably after or within a longer drive.
If this can not be carried out for whichever reason over a longer time, the DPF will irreversably get damaged, and you have to replace
- the DPF
- the turbo if you're unlucky (and the b57 has two of them)
- and the so called swirl flaps which control the amount of exhaust gas being recycled at a time.
Search the web for what this all is and does. Replacing these parts costs A LOT (really A LOT!) of money.
The distance for this problem to occur may be 100000 Km or less ... the mileages vary.
On many BMW Diesels it is possible to disable this system, by either reprogramming the respective controller/s, mechanically closing the tube for the exhaust gas recycling or manipulate the swirl flaps unit. This is illegal, I have heard ;-)
But it protects your engine and saves a lot of energy for the production of spare parts and preserves your car from burning additional fuel.
I don't know whether the newest engine versions are "improvable" in the way I have described. But I am convinced that there will always be people which find, well, a solution. ;-)
Lacking a DPF, petrols don't have this problem.