I have had very good luck with TSM running on Win2000.  I am one of those
people that believe that if you throw the hardware behind it will work.  I
would take into consideration what the majority of your clients are.  If the
majority are Unix then go with TSM for Unix and if the majority of clients
are Intel go with Win2000.  You will see better throughput going to "like
boxes".

Like I said I run Server 4.2 on Win2000 with a database size over 75GB.
Incremental data is about 1.3 TB of incremental with Monthly archives
totaling 5+ TB.  230 Clients, 54 SQL Agents, 10 Exchange Agents running.
Majority of clients are Intel plus 20 Unix (multiple flavors), 1 AS400, 1
Tandem, 24 Macs.

Server is Dell 6450 Cluster with 1.6 TB of fiber attached diskpool, 2
powderhorn STK libraries with 10 Fiber Attached 9840 Drives 4 in one library
for copypool and 6 in first library for tapepool).  Each Cluster node has 2
GB of memory and is on a Gig network switch

No problems completing backups within backup up window of 11pm to 6am.

Ron Pavan
Sr. System Eng.
MGM MIRAGE