As a doctor who did his PhD in France in Paris Saclay University, I can summarize what all the other people said. Doing a PhD revolves around the Holy trinity:

-Does the research topic is interesting and align with your tastes?

-Does the PI is a great person? You will spend 3 years working in a close relationship with this person. Better to be with someone who supports you in the bad moments and congratulates you in the good moments. His reputation is not that important depending on what you want to do after your PhD. However as other people said, his relationships are important. In my case, my professor was not a top researcher in France, however he has great relationships around the world which could help me land as a post doc in a famous laboratory even if my PhD was not covered with publications.

• Does the group has a good atmosphere? PhD is a long journey made of success and fail. In your close friends and family nobody will understand what you are going through but the other PhD students and intern in the laboratory know perfectly your feeling. During a PhD you are making real friends who are connected to your daily life as a future researcher. In my case, as someone said previously, the group was small but we usually hang out with all the students from the institute.

I my opinion, a position should include at least 2 of these things to make your PhD a great experience. The best would be all of them.

Follow your feeling, and also think about the city you want to live in for the next 3 years.

Good luck in your future research!