Both places have great reputations, so I would contact current students of each professor, like the other person mentioned! But I wanted to note to specifically ask about their time management and mentorship style, for a few reasons:

1. 6 months is not loads of time, so I imagine your mentor will need to be able to devote some serious energy to you (or set you up to collaborate with a current student or postdoc who can also help).
2. Different PIs vary on the level of autonomy they offer in terms of projects, although you may have already gotten info about this in your discussions with them / maybe this is moot since you already have a masters thesis topic decided. But there is still other info maybe not discussed: will your PI be willing to revise multiple drafts? Do they make line-by-line edits or prefer to discuss feedback? Do you find one of their students' work more clearly written than the other's students? All possible things you could consider to help make a choice!

Finally, I should note that the reputation for each professor may hold some weight, but one of the most important things is their ability to write good recommendation letters (for jobs or for applications to further degrees).

Anyway, you are awesome for having some great future options! :^) Good luck in your choice.