It's not unheard of to hear back on April 15th, and in fact, I'm almost assuming that's when I'll hear from the school I'm wait-listed at. My recommendation would be to try to arrange a visit to Northwestern and have your decision in mind in the case you do get a last-minute acceptance. I'm kind of in the same boat as you - I've been accepted to one program and am wait-listed at another, and I will have visited both programs by the time April 15th rolls around.
Good luck! I did my BA at Northwestern, and from the perspective of an undergrad at least, the history department was fabulous!