Segment per segment it is unfair to compare Alfas against BMWs. They are both designed to different motoring standards and engineering philosophies. You can put the Alfas in the Peugeot or Renault grouping. BMWs are in premiere Merc, Audi and Jaguar group.
If UK’s scene can be a guide, the cost to maintain a 156 is about 75% more than a similar performance BMW 3. Why you may ask? It is due to the better reliability of the BMW. JD Pwr research give only 1 star to the 156 while BMW 3 gets 4 stars.
Japanese cars’ strength is in the light duty electrical and QC. BMW strength are in robust designs of mechanical parts and heavy duty electrics. For example, take a Honda Accord 2.4 and a BMW 320i. Thrash both cars very hard continuously say for 5K km/month for 2 years while keeping their service intervals about similar 15K-25K km. You will see an extremely tired, loose and power-lost Accord, while the 320i will still be tight, as strong as ever and ready for more fun.
In so far as metallurgy comparison goes, take 3 cylinder head bolts. Each from an Alfa, BMW and Honda. Torque them to breaking points. See for yourself which one snaps 1st . Last will surely be the German one. Or try to source for a very high pressure fuel injection system, say of more than 1,500Bar/22,000psi working pressure. More likely you can only turn to a German one like Bosch.
So if you drive very hard and want to keep your cars for, say 300K km before overhaul, get a BMW. 6 cylinders would be better.