Thanks for your question. Before answering I'd like to say that there isn't a defined "right choice" but there are considerations you can take in order to decide between the firms. I'll list some of the main points I considered when choosing an internship and add insights from other people I've known. Note that these are not ranked in order or priority.
1) City / location: depending on the location of your staffing, you'll get different types of work and interact with different clients. Typically, you can research the main industries in a given sector or ask the firm about the main projects they take on along with the scope.
2) Culture / fit: this is often mentioned but more than just getting along with people it's about considering the dynamics of the job (top-down, horizontal, bureacratic, open-minded, etc.). Although you won't always get a full insight by networking with people within the firm, you will be able to understand project dynamics and understand the expectations for interns (and then Analysts or any equivalent roles). If you look up firms online to get a sense of their culture, I would advise you to be cautious and consider taking things with a grain of salt. For most firms, cultures vary on an office-to-office basis, so the NYC office may be drastically different from the Houston office as an example.
3) Rigor / learning opportunities: How challenging is the job? What is the scope of the projects? Are there learning opportunities outside the projects? These are some of the key questions to ask yourself not only for the internship, but for a full-time opportunity, too (remember, the end goal is to determine if you see yourself as a fit for the firm long-term). The more learning opportunities you have, the more engaging and satisfying the job will be (plus, you'll probably have more opportunities to work elsewhere in case things don't work out).
Hopefully this helps. I'm sure I didn't cover all considerations but these are the top ones I would look into. If you have any comments or questions feel free to let me know!