I work for a government based organisation here in the UK, we are about 400 strong and I have just been through the entire process that you're going through now. I had a look at products like Altiris, Kaseya, Prism Deploy, Symantec Ghost Solution Suite and SCCM. I finally got it down to two products, Altiris (including the Rapid Deployment Pack from HP as we already own this) and MS Config Mgr. In the end it all came down to what would work in our new infrastructure.
We are going to be migrating to 2008 server from an NT4 based domain and to put it simply Altiris simply won't work in a pure 2008 environment, you can't install the current version 6.9 and at the time of asking Altiris couldn't provide me with a release date for version 7.
Since making the decision to go with Config Mgr I have spent a lot of time using the product, been on a course and even sat the Microsoft exam for it. I have to say I am converted, I just love the product, sure it has it's quirks (bare metal builds until R2 were a bit of a nightmare) but all in all it's exactly what we need.
I would say however that in your environment you wouldn't need to go for the full blown SCCM product, you should find that SCE (System Center Essentials) is more suited to you.