RoundCube is quiet new.
Developers developed Squirrelmail for a longer period.
For these reasons Squirrelmail is probably more stable.
I have to say that i do not have bad experiences with the latest RoundCube version.
You can use both .pkg files which you can download from http://www.ispconfig.org
If you do this, Squiirelmail or ROundCube will run on port 81.
As an alternative, you also can use my howtos to setup Squirrelmail or RoundCube.
If you are interested in these howtos, have a look here: http://www.ispconfig.org/documentation.htm
With this option, you can run these webmail clients in the webspace of your choice running on port 80.