An important consideration is the length of each program.
At Harvard, the AM in Statistics requires that you take 8 courses, 4 each semester. That means that you start the program in late August and you receive your degree in late May. The problem with this is the absence of any internship opportunity, because many firms like to select their permanent job candidates from the pool of prior interns. This gives them a chance to "try out" the candidate before extending a permanent job offer. Think of a summer internship as a paid, 3-month long job interview where you are being assessed daily.
At Columbia, the MA degree requires 10 courses. While it is possible to take 5 courses each semester, finishing the degree in one year, the problem with doing so is as above: no opportunity for an internship. However, at Columbia, you could also extend the program to 3 semesters, simply by taking fewer than 5 courses each term. For example, you could take 4 courses in each of your first 2 semesters, seek a summer internship, and then in your final (autumn) semester you would only need to take 1 more course (if you obtained academic credit for your summer internship) or 2 (if you did not receive academic credit for your summer internship.)
Obtaining academic credit for an internship generally depends on whether you are in the USA on a student visa or if you are a US citizen or permanent resident. If you are a foreigner then you may be required to register (and pay tuition) for a "curricular practical training" course in order to be able to pursue a paid internship under your "F1" visa restrictions, while if you are a US citizen or 'green-card' permanent resident then you do not face any restrictions on your ability to work in the USA and you do not need to pay tuition to your school for the "privilege" of having a summer job.
Note further that most firms operate under the following recruiting calendar:
Candidates for full-time positions starting the following year are generally interviewed in the fall, e.g., in October. Indeed, the annual National Financial Mathematics Career Fair takes place each year (at NYU) at the end of October (this year's event is taking place
TODAY.)
www.finmathjobfair.org/
So, if you are pursuing a one-year degree, you would need to start interviewing for your full-time job almost as soon as you arrive on campus, which means that you would be at a disadvantage vs. students who have already had a year of studies under their belt -- you would be less likely to succeed when peppered with the same hard interview questions which firms would expect you to be able to answer. Also you wouldn't be able to show any grades from your graduate studies, while other people would be able to show a full year's transcript.
On the other hand, interviewing for summer internships generally takes place at the beginning of the second semester, e.g., in February/March. At this point you would have already had a semester of very intense studies, and would be on equal footing vs. other candidates for summer internships.
The other thing to consider is geography. There are many excellent buy-side firms in Boston, but the population of firms in which you are interested in working is larger in the New York area. It would be a challenge (though not impossible) to be running back and forth between Cambridge and New York on job interviews, though that would interfere with your studies (except during Harvard's extended January break.) You would certainly find it more convenient to hop on the New York City subway to attend a job interview vs. having the added travel headaches of coming down from Boston.
I'm sure that you would obtain an excellent education at either school. However for the logistical reasons outlined above, you might find Columbia better suited to your needs, particularly if you choose to extend the program through 3 semesters rather than 2.
Good luck!