I downloaded Poseiden and tried it. Poseiden
has more features than ArgoUML. I thought
the documentation was poor. I had trouble
figuring out how to show aggregation. If you
search the OO,
Patterns, UML and Refactoring
forum you'll find somebody else came to the
judgement as I has on this subject.
Now don't get me wrong. I think an open source
UML tool is a great thing. I think people
should use this tool and contribute to its
development.
If time is important to you in finishing the
SCEA part II exercise, I think the trail version
of MagicDraw is a better choice.