Along with what she said, I'd recommend just getting an idea of what programs/colleges you want to get into. Then look at those programs. You need to know what it is you want to get out of your program. Do you want to do applied work or have a theoretical basis? Do you want to do a different PhD program or teach or go into the workforce? Different schools have different focuses, and usually that is outlined in their programs, course requirements, and whatever special groups they might have (e.g., UCD has a biostats group and an applied math group in their math program, because they offer many courses that support biostatistics or applied mathematics, respectively). If I wanted to, say, study more theoretical (abstract) mathematics, then Berkeley would be much better as they offer many courses UCD just does not (e.g., Lie Groups, Mathematical Logic, and a ton of different algebras whereas UCD abstract maths focus on combinatorics, topology, and analysis--they have utility in applied fields even though they're abstract).