I also received offers for both programs (NYU M.S. in Mathematics / Columbia M.S. Applied Mathematics). Think it depends on what you want to do, so I will just give my $.02.
I'm interested in quantitative finance, so I plan to use the degree to build background in PDE's/machine learning/statistics/coding. This is probably easier to do at Columbia since you can take half the courses in CS/op research departments, though it's possible to do it at NYU as well through research projects and the optional thesis. NYU could be 4 semesters full-time if you write a thesis, Columbia would be 3.