Sixt's pricing are very sensitive to demand; much more than competitors. Rates tend to drop starting 10 days prior to pickup and may tick back up a couple of days before pickup but often times it drops even more. I usually re price my Sixt reservation even an hour or two before pickup.

You should also compare with Sixt.de and Sixt.co.uk. Usually a lot cheaper than Sixt.com.

Most, if not all, U.S. credit cards offering auto rental coverage will be primary coverage in Europe. No need to go to AMEX premium car rental unless you are renting something more expensive as it covers up to $100k.

If renting a 3er or 5er and their BMW/MB equivalents, any card offering coverage should be fine but you may want to call your bank to confirm specific vehicle exclusions. If you are renting something like a M5 or exotic, Amex premium coverage may be the way to go but I would also confirm with AMEX first.