go with UNIX.

I think some administrators that come from a Windows background are hesitant to switch to a UNIX variant for fear of the unknown.  In the environments I have managed, the UNIX hosts outperformed the Windows servers in one large way - the need for reboots if something is wrong.

It is fairly rare to have much downtime on a well tuned Solaris 10 machine.  Reboots are only needed when major configuration changes are made, and they rarely shut down on their own. WIth Windows on the other hand, something small goes wrong and you could be pinging a blue screen for hours until you log in and find that Windows didn't like something and bs'd.  I've only seen one Solaris master server become unresponsive, and that is due to a kernel panic stemming from a patch that was made by one of the SA's. 

Windows 2k8 may be different though, and I'm not going to knock 2k8 until I try it, which may be never for a master server.

but then I come from a UNIX background anyway & couldn't justifiably comment personally on NB in a Windows environment. There are members out there tho' that could probably give a more balanced answer, dealing with both set-ups on a day-to-day basis.

At the end of the day it all depends on what you would be most comfortable with - if you (or your admins) have had no exposure to UNIX then I wouldn't go that route & the reverse is equally true (almost).

Netbackup is born as a UNIX application (Backup Plus hence bp) and I think is there the main development effort is laid. For larger environments Unix master servers tend to be the recommend platform. Larger > 5 media servers.

I have had the privilege, no the pleasure, no the horror of doing both.

We started with a windows master and 2 windows media, and 1 UNIX media ( for some strange reason management seem to think that the UNIX (aix) servers had to be backed up by a UNIX media server (also aix).

We were always fighting the Windows media servers and the master. Rebooted them a lot.

When we upgraded to 6.x we went to an aix master/media and 1 aix media ( also 2 windows san media servers just because they were big).
I now have very few issues, only reboot when  I upgrade (not needed just do it because).
with the master being UNIX even the two san media servers give me little problems. 
But help me if I have to change a tape drive - Unix cool - windows have given me a little fight a couple of times.

windows master - you cannot push to UNIX clients from the console
Unix master - you can push to UNIX clients from the console, and push to windows clients from from install media (except for the new 2008 servers - for some reason that does not work)  Which means I have all my UNIX servers upgraded in about 15 minutes with out have to log into them.

I feel I don't work as hard has I use to with the aix master.  Not to say I don't have problems some times, but it just seems to be easier.  No registry to mess with, the bp.conf file is on disk and you can look at and and change it easy.

(I should also admit I am a UNIX admin - that learned a lot about windows when we had that master)

Do not join the Dark Side Luke, stay on the light... that is UNIX against Windows, Windows will keep you in the dark when issues came by where UNIX is clear and always more reliable.

There is nothing to think about here...

you may have an easier time configuring new devices into the OS.  I dont know of any other advantages.

Have the pleasure also of administering both Windows Master and Unix Master. But our own NBU environment has been on UNIX since NBU 3.x days, never once considered Windows.

UNIX is mostly always up, only time you need to reboot is for kernel upgrades. Security updates just go in for the most part. Just think how much pain is shutting down a NBU master in your backup window.. every "MS patch Tuesday"
What I would say if your backup environment is complex, then scripting on UNIX is so easy, and I am no programmer. And the available UNIX scripts out there FAR outweigh those for Windows. The scripting helps get more out of NetBackup as an administrator and batch scripts aint much fun, unless thats your thing.

What I'm saying at the least have the master as UNIX.

you'll be much more likely to find help and expertise in the UNIX world (I'm not saying that Windows admins are not experts, I'm just saying that I think there are many more UNIX NBU admins out there) and you could probably get help easier when it comes to UNIX issues.

I have Solaris SPARC, RHEL 4+5 and Windows media servers.

The solaris servers are the most reliable. The linux servers are reliable but i have a few that have to have their tape drives readded every so often (too rare to really troubleshoot).  The windows server are required because of VCB for VMware but my main gripe with them is the TCP stack.  Network backups on windows are just slower than Unix.  I am curious though how this new 2008 server i just deployed for a new VMware environment will stack up against the unix boxes.

Just my 2c.

Unix gives you the performance and the ability to script a lot of the day to day tasks.

I'm a Unix guru and I would warn you that if you are the least bit squimish about Unix, you better stay away from Solaris.  I would only recommend a Linux implementation if you're a newbie.  I've run NBU on all of the major platforms and here are some pros/cons IMHO:

Solaris
CON - Solaris is the toughest to manage and not for the faint of heart.
CON - sgscan and build sg is just a pain and you have to do it everytime you add any devices
PRO - SUN hardware is quite affordable and scales very well.
PRO - The benchmarks on performance are pretty good, I would say the best of the 3 OS's mentioned here.

Linux
CON - There's a lot of info on the WWW, beware of misinformation about config etc ... know the specifics to the release you use.
CON - You can stuff with a GUI ... boo! :)
PRO - Super easy to figure devices, Just make sure you're on a 2.6 kernel ... stay away from 2.4
PRO - Cheap to support, hardware can be cheap too, just make sure to do your research about compatibility

AIX
CON - Expensive as all heck
CON - Stupid /dev/ovpass stuff!
PRO - Very simple to add devices
PRO - smitty is actually a nice menu driven config tool which will show you the commands you run.

Additionally, just to reiterate what others have said, Unix is the way to go when it comes to scripting.  In that vain though, I would go with a good scripting language like Python.  This way if you need to port to another OS (i.e. Windows bleh) it's very easy.  But if you must use a shell script, which is quite easy but doesn't have all the goodness/functionality as Python does, use bash as your interpreter.

BTW - Start here http://www.symantec.com/business/support/overview.... and check out all the compatability information (OS, Hardware, etc) if you haven't already.

Good luck!

CONS for Solaris, but don't know whether I should as you quote one of my most favorite lines from a film (unfortunately I can no longer do the voice!)

Find Solaris a doddle to manage (don't need to do anything), sg build stuff can be a pain but once it's done ....

But then again, it is what you know at the end of the day & the learning curve can be steep if you don't!

Again, no mention of Windows!

Again ... just an opinion ... wait until you manage multiple VTL's robotic control and DirectCopy (NDMP) to dupe ... then you will feel the pain of sgscan (oh and never have the GUI restart the media managers services unless you need to take a 30minute+ coffee break).  It took me the hard way to find out that LTID has a master server device limitation of 128 tape drives, Yikes (BTW- I hear that is resolved in NBU7).

I will mention one thing about my experience with Windows, which is limited to media server implementations.  With Windows 2000, you have to reboot to get tape drives added, Windows 2003, no reboot required. Otherwise, I know not what Windows can do for me. :)

Always do your research before implementing (hopefully you have that luxury) ... it will be a life saver.

Ouch!