The issue here that the user asked about was related to scalability.  The
reality is a Windows Server will only scale so far and a UNIX server will
scale about 10 times as suggested.  

The real question to be asked is how often does your rabbit farm get
pregnant and add more servers.  If you have AIX experience onsite to support
the OS then I would go that route.  TSM administration is nearly identical
through the ADMIN Browser GUI on both.  The amount of UNIX work that you do
is relatively small.  If you have no AIX expertise onsite and a reasonably
small number of client machines (desktops and servers), then Windows would
be your platform.  I nearly never log onto the AIX server or Windows server
except to delete storage pool volumes on disk.  Everything is done through
the ADMIN command line or the GUI.

I am a mainframe person, either AIX or Windows are easy as far as I am
concerned.  We have both.  And like I said there is not that much system
level stuff to do.