I am currently in the following situation: Another person (not from my department) and I did a study together. The other person is an associate professor, I am a junior researcher (medical doctor). We performed a secondary data analysis of clinical data. The data is his and I performed the analysis. We both wrote the manuscript. When discussing about authorship my collaborator was very liberal replying "XX, it is really up to you. Take the first or last authorship, whatever you prefer and like best". We are both from different countries (in Europe). The manuscript contains an author contribution section, detailing who did what.
How would you handle such a situation? I read that this is field-specific so I was wondering what would be best for medicine / statistical science.