Depends on what you are trying to do and in what kind of environment. If you are just trying to do a small workgroup server, Leopard will fit that well and possibly in a quicker, easier fashion. If you are looking for a more advanced configuration, both servers can play in that arena, so it depends even further on what you are trying to accomplish.
As it was mentioned, you get the ability to look into Exchange with Windows, but there are additional costs for running, supporting and connecting to that. There is also a standards based email server and collaboration service in Leopard Server. Those services are bundled into the software and will not incur additional licensing considerations.
All depends on what you are trying to do and what technologies you like. Do you want to run a Windows OS or a Mac OS? Do you have any hardware investments to consider? What clients are you going to support? What services are you interested in? Evaluate some of those considerations and you should have a better idea of what platform you might want to use.
Michael