I would also suggest that you do the math concerning the impact of the accuracy of your crystal on the timekeeping ability of your µ-controller, and compare the result to a similar calculation concerning the frequency of the power grid. This is just my guess, but this sounds like a homework assignment, and such a calculation in support of whichever approach you choose will definitely earn you points with your professor.

Good luck

P.S. If this is a homework assignment, you will want to be careful not to ask anyone on the forum to actually do the assignment, as in the case of asking for prewritten code (see the forum rules).