Britax one4life vs diono 3qx

Hi Laura, thank you for your inquiry, Both the Diono & Britax range are US safety rated so can’t be used in Germany, you would need a European (ECE) seat for this purpose. I have added a link here for the ones we have available: https://www.babyonthemove.co.nz/shop-by-standard/ece-standard?page=1.
To answer your second question, The Diono is the slimmest seat that we have, however, the Britax One4Life is smaller in height so will give you possibly more room in the passenger seat. I would actually recommend contacting your closest store in regard to this as they will need to know the model of the vehicle you have as well as it does make a difference. I have added a link to our store locations here: https://www.babyonthemove.co.nz/store-locations/