Check Point vs. Microsoft’s ISA

Our company is considering to upgrade our exsiting Check Point firewall to 4.1 or to implement ISA. We are an NT/200 shop and some upper management thinks your company should stay with NT products but I do not feel this is a good enough reason to switch to ISA. I do not hear good things about ISA as a product to protect your internal network. It sound like a great proxy/cache server. I know there are pro’s and con’s to every product. But which would be the best choice in this situation?