There is several diffrent products out there, and basically it all boils down to two factors. Do you need all the features that program a has over the rest and is it worth the extra money!?
We tested Landesk and SMS2003 before we settled on using GFI Languard which does the job for us, it is more of a network security tool than patch management software but it does what we need and it was reasonably priced to.
Landesk seems to have won quite a few magazine tests and initially we looked to get this product as it does a lot more than just patching machines but in the end we found it to be rather expensive and had a lot of features that we didn't have any need for.
So if I was in your shoes I would try and get hold of a few tests, then talk with your collegues what you need in ways of features.
Then download a trial versions of the software options that suit your need and after this make your decision on what software that suits your need and fits your budget as with 12k machines it could be a rather huge price depending on the pricing structure of the software.