That's a complex question.
In general a letter from a department chair would hold more weight. However, it is uncommon for a chairperson to do a lot of clinical work, and you might not even see him/her during your month.
In general PDs tend to do more clinical work, so there would be more opportunities to work with one.
Another option for letters includes any attending that you worked closely with - just because an attending isn't the chair or PD does not mean he/she is not a big name in the field.
Of course this general rule applies as well: the best letters are from those that you worked closely with, that know you well, and that will write strong recommendations.
A lukewarm letter from a chairperson or PD or a letter from someone who you didn't work closely with will most likely hurt your application, not help it.