Some further thing to consider:
-The 200 EUR Sixt voucher is not a voucher for Sixt rent a car but instead for sixt ride (similar to pre-booking an airport transfer). Only up to 20 EUR of the 200 EURcan be used for any single booking.
-Even without any bonuses your earnings will be lower for the german card. You earn 1 MR point per EUR spend vs. 1 per USD spend (~10% difference)
-Not only are the transfer partners different, the memberships you receive with your card are different as well
-The german card has a 2% forex fee, the US card none