I've only used A1 and Magenta (T-Mobile Austria).

When I roamed with A1 in Germany, it dropped to pretty slow 3G. Maybe it was where I happened to be not in a big city but a small village in the mountains.

I noticed that Yesss is a subsidiary of A1.

So I wouldn't base the decision solely on roaming.

As for registration, yeah if you feel confident possibly using a German website. The other thing is it may not take US credit cards. That's why in September, I just went to a Magenta shop and had them take care of it.