what if the big bad budget wolf says no clusters though?
All things being equal (availability, processor power) a cluster will typically be the cheaper option.

And when would you personally deploy a Unix based OS over a Windows one?
I can't think of a situation where I'd prefer a UN*X server... by server I mean typical http, ftp, smtp, dns, rdbms, etc. UN*X tends to have a higher cost of ownership, less security, comparable availability.

And what about Mac OS Server?
Meh, not until the system impresses me a little more.

I get the 5 9s on my LAN and I give it a pretty good sized load
It's one thing for a small LAN, quite another for a network with tens of thousands of users or a server with tens of millions of user. :-P

cheers,

catch