NT is popular because people have heard of it.
Blue Screens, DLL Hell, Rebooting, etc are MS issues that really have no equivilent in Netware or *nix.
You are absolutely correct that Novells security is better than NTs. Windows 2000 tries to copy NDS, but it does a poor job, and Microsft has decided to leave the bug fix for at least one of the flaws until WinXP.
MS is basically a mediocre solution for every circumstance. It excels at nothing, but a trained monkey could click their way through an install, so it seems cheaper to management.
Your best bet is to install ZENworks (even just the free starter pack that comes with the new clients) MS has nothing that can come close to that, and dollar wise, it will save your companya fortune. That will convince your management to stay with Netware far better than a technical argument that many techs don't even understand (never mind management).
Plan for twice the support to be needed for an NT shop as for a Novell one, simply due to the flat file structure of their users. 2000 is better, there's no doubt, but it still isn't a 10 year old product that wins Best Of awards every time it enters a competition... Look for examples of people usiong MAD. There are none. NDS, on the other hand you already know and use. Why spend money replacing a good solution when the economy is entering a recession?
That's my thoughts...
Kev