Here are some things you could think about in making your choice:
- Research fit. From your description, it sounds like UCR is very good at one thing while Brown might be more general. So, UCR might be better if you really want to work in that thing, but if not, Brown might have more options for you.
- These two schools are also in very very different locations. So don't forget to factor in what type of city/climate/culture where you would be happier and thrive more. Check out the cost of living vs. stipend too and see if you can have a happy life.
- UCR is a public school while Brown is a private school. In the USA, there is often huge differences in the resources available to professors and their students between rich private schools and often-struggling public schools (note: I don't know anything about UCR specifically in terms of finances, this is just a generalization). So, I would talk to students and professors at both places about the resources I would want to complete my work. For example, you might ask potential advisors and students in their groups questions like, "how often you present at conferences domestically/internationally?", "do you collaborate with people outside of the university?" (good for building your network), "are there any travel grants or research grants available for grad students?", "what equipment/resources are available to students for doing work?" (e.g. do they buy you a new computer when you start? give you money to buy one? etc.).
- Talk to professors that you are specifically interested in at each school to find out what they are like and whether you would get along. Talk to the students in these profs' groups to find out what it's like to work for them.
- What does your funding package commit you to? Usually, it is preferable to be funded by a fellowship or a research assistantship grant from your advisor, but I know TAing is very typical for theory fields. If you are TAing, what is the TA load like between these two schools?