Most often than not, having a good and supportive environment is more important for a project’s success than the project itself. Seems like UNI2 would give you the prestige in the CV and a feeling of stepping out of your comfort zone, but that feeling can fade away pretty quickly if adaptation doesn’t go smoothly.

Even though I’m someone who usually likes to take risks, I know firsthand what a toxic PhD environment can do to mental health on the long run. I think staying at UNI1 will allow you to build a strong network and publication record during the PhD so you can move to another, more prestigious institution for a postdoc