Tough call, as both cities can be exciting for students. I’ve lived in both places and also studied in London - Paris is more exotic for Americans and fluency in French can be quite helpful professionally but my impression is that London is more diverse/cosmopolitan and more of a crossroad internationally (of course, I am sure some people will disagree).
Both cities are generally safe, although I feel there are more day-to-day hazards in Paris (e.g., metro pickpocketing) - manageable if one takes sensible precautions.
But if I have to choose, I’d pick Paris - it is more compact and accessible, provides opportunity to become fluent in French, and is easier to travel to other parts of Europe. Living with a French family also looks quite appealing, including from a parent’s perspective.
I am sure what I am about to say will offend a few people on CC but, as a long-term expat in Europe, I’ve observed certain cultural traits/behavior displayed by some Americans that don’t play well in Europe - speaking loudly, being impatient with speed of service (particularly in France), making stupid jokes such as why do the British drive on the wrong side of the road, etc. Avoid that, and things will likely go more smoothly and the experience will be more enjoyable.