Don't mean to sound critical but if your wife has two cars does it matter that the car isn't as practical?
One should have a fun car in the household and M8 is still quite practical apart from the rear seats. Not like a McLaren or Ferrari
Different story of course if the car causes friction in the family...
That said, I think the M5 is a fine car but a step down from the M8 and somewhat more boring looking. But then I think sedans are just not that exciting.