Having lived in Germany, I'd say neither one. B&N and Amazon were the two most stringent book stores for enforcing geographic restrictions. A lot of the books you might want to read are not available, in English, when you live there. There are workarounds that usually involve a VPN connection that will give you a US IP address on your computer. (HotSpot Shield will do it for free with advertising, but Firefox and AdBlock Plus will remove the ads.) Some stores just require that you have an American credit card or address. I'd certainly hang on to an American credit card while you're there. There are several book stores selling ePub format, so if you can't get around one store's restrictions, you can try another. Library books are available in ePub format. I'd suggest that you see if a library near you offers electronic books (ask about OverDrive). My old library was happy to continue loaning me books while I lived there.
B&N will NOT support their Nook outside of the US. If it breaks in Germany, that's not B&N's problem. I'd recommend looking at the matrix (
https://www.mobileread.com/eink/) to get a better idea of what's available. Some of the less well known brands are sold and supported world wide.