Go with the local SIM. Most of your time will be in Germany? When we did a trip to Vienna and Munich last spring I found it very easy to get a SIM for my iPhone at a T-Mobile store in Vienna, I just walked in and handed them my phone, they put a SIM into it, made the whole thing work and I paid some amount for the data plan. I then could share the hotspot with my wife.
Then when we got to Germany we repeated the process. I was not successful in finding a reasonable solution that covered all of the countries.
Scott