WinNt.also refer sites if any.
Unix flavors tend to have less downtime, but have a higher learning curve.
Unix works very well as a server. NT still has a few areas to catch up, many of which are addressed in 2000. But the best way to decide is to do field research and that means installing one or the other. In some areas NT outperforms Linux and vice versa. You must determine your reqirements list before anyone can help you.