Not all *IXes are equally stellar. But most of them (even Linux) are an order of magnitude more reliable than Windows. Sun would assert, with some justification, that its Solaris brand of *IX is two orders of magnitude more reliable. *IXes are generally easier to secure, too, since (a) there's a long history to them; (b) they're well and widely documented; (c) lots of (free) administrative tools are available; and (d) there's more granularity, i.e., you can disable functions you don't need, log things you want to log, etc.
*IX generally performs much better than Windows on equivalent equipment, though if your application isn't pressing the frontiers of what's possible you can buy your way to adequate performance without too much trouble.
There may be reasons unique to SyteLine, which I don't know, to run it on Windows. And if your administrators aren't familiar with Unix, you have to weigh the cost of the learning curve vs. the reliability gains.
