If going to Exchange 2007 - Windows 2003 or Windows 2008?

I had a customer ask this very question, and I replied with this. Feel free to comment and add any pros/cons you can think of.

Pros of deploying Exchange 2007 on Windows 2008:

Quicker deployment time � Win 2003 requires IE7, IE8, SP2, and about 55 patches to get it current. This speeds initial deployment, but more importantly in a crisis when you need to redeploy, you won�t need to wait as long for patching procedures. Additionally to this point, 2008 keeps the OS CD installed to a hidden directory unlike 2003 that will prompt for a CD when you add/remove features like IIS. Windows 2008 also has command line deployment for most Exchange pre-req�s so that spin up time is pretty quick.
Longer solution shelf life � Windows 2003 support lifecycle policy is to end support 2 years after the last Service Pack. SP2 for 2003 released in March of 2007. So unless we see SP3, at this point, Windows 2003 is already out of support. We typically attempt to deploy solutions that last the 3-5 years most organizations amortize their hardware.
Less known bugs/exploits available � granted, with newer OS, there is an inherent risk of a newly found exploit being found, but there is a LOT more know about 2003 code now.
Options to change after are limited � if you go 2003 now, and later want to move to 2008, there is no in place upgrade of the OS on an Exchange 2007 server � it would be decomm a server, rebuild as 2008 and repurpose. Far easier to start off at the end point of OS.

Cons of deploying Exchange 2007 on Windows 2008:
Cost - Your first Windows 2008 server means you need to migrate all of your CAL's to Windows 2008. For smaller organizations, or anyone with their CAL's on a Software Assurance plan, this is VERY easy. If this is an unexpected expense, Windows 2003 may be your only path.
Newest stuff - If you are of the "Wait for SP1" mentality when it comes to Microsoft products, you likely will have a hard time with this decision.
Missing skillset - If you have never run Windows 2008, there is a slight learning curve. And I do mean slight - most things are VERY similar, but if admin skills are a concern, sticking to what you know may be appealing.


Again, PLEASE comment, I know this is controversial for some folks.