This is not really an easy question, as the applications run in different environments. I would say you best bet would be to develop in Java. Java code should have no problems running on both systems. Your next best bet, would be Mono. With Mono, you can develop for windows and run on MAC. However, there might be a few things you can't do with Mono, and you will probably have to target the 2.0 .NET framework. With either JAVA or Mono, you will have make sure the installs have the required framework.

Good luck with you project.