I could send you to Microsofts site, to check out the advantages of WIN NT versus Novell. A lot depends on how many users you are talking about? Do you have an MIS person or team? Do you want to spend a lot of money in changing?
Both networks have advantages and disadvantages. A lot of Microsofts server products will only work well with Microsoft Server. If you are not changing software other than the networking software, I would say stay and avoid alot of headaches in changing over and learning a new network system.
Hope this helps.