Active directory Operating System

hello all..

i’m a new employee in a very huge company.. i work as an IT support.
we want to migrate our 2000 AD, but i have to do a research to find out wether to migrate to 2003 AD or to Longhorn AD.
so what are the tools that i can choose to make the right comparison , or what are the questions the have to ask to determine which os to use??
can i find a job aid software on the web that can help me regarding this issur?

thanx alot.