I understand that you do NOT want a minivan, but... a minivan will have about the same size. It will have 3 separate seats in the second row, which will allow you to easily mount 3 baby/child seats. To do that in 520 will be a BIG pain, especially that you need 2 much bigger ones. Especially the 9 years old will have problems with the belt to fasten it easily having another seat next to him/her. In a minivan the kids will have plenty of space to move (walk) between their seats and front seats, so getting them in and out of the car is going to be much easier. Sharan has sliding doors, and is higher so putting kids into the car is much easier especially in tight parking places.
The boot in Sharan will be MUCH bigger. To get 2 trolleys into the boot + all the luggage for the vacation is a big challenge. With a minivan you will let your wife take whatever she needs (or not
).
So the bottom line is: buy both: 520 + Sharan
. BMW for fun, and Sharan for practicality.