It depends on what properties you are interested in observing. I have used both Comsol and OptiFDTD for a similar application and I have found that OptiFDTD is simpler, more effective and less ambiguous in its results (closer to expected output). However, I am only analysing transmission spectrum over a range of swept properties (such as refractive index), and also the E-field distribution. Another advantage of OptiFDTD is that you can watch the temporal evolution of the model as the optical waves propagate - this is useful for examining factors such as coupling between interfaces and materials. Comsol is more powerful and comprehensive however, if you are confident you can set the model up very accurately and succinctly (there are a lot of properties to check before running a COMSOL simulation).