Yes, I expect the 5 series to depreciate more than the 3 series so servicing costs would be fairly non relevant compared to the depreciation differences.
The insured warranty for my E60 was £72 per month (no excess and without roadside assistance), but was just £41 for E85. The warranty cost of E85 is similar to E90.
Thought BMW did a 5 year service plan for a very reasonable amount, so even if you were to do silly miles & exceed the max milage of the plan would still be a good buy