NT is great for doing one or two things very quickly, but UNIX will generally give better performance when you put 2,000 users on the box.
We use NT or Win2K boxes for many of our DSS systems. The reason for this is price/performance. These boxes have to summarize 200 million rows of POS data each week in a very small processing window. During the day, there are only a handful of Discoverer users accessing this data. An 8 way Dell machine fully maxed out with 16GB of memory and a couple of RAID arrays with 1TB of disk space is only around $50,000. To get a Sun or HP box that could do the processing in the same time, you'd be talking hundreds of thousands of dollars.
However, we use HP UNIX for many of our OLTP servers. On these machines we have hundreds or thousands of users so throughput and 24/7 uptime is essential.