new module or branch?

Hello,

I know that this is probably a configuration management question, but...

We have a situation where we need to make a copy of an application for 
another division. The division will use this new version of the software 
for at least the next several years. At some point in the future, we may 
  attempt to get everyone back on one version.

So the question: should we create a new branch, and have the other 
division work from the new branch, and we keep working from the main 
trunk, or should we copy the code to a new module?

Any advice is appreciated.

Tom