Good morning,
For a full answer I would suggest that you tell us a bit more about you web site.
For example if you have a lot of database access then any efficiency advantages in the OS can be squandered by the database.
MySQL, PostgresSQL and
MS SQL Server do perform differently depending upon what you are doing.
There is limited advantage in having the OS stable and still running if you have a lot of program code, (PHP, VB.Net etc.) that is buggy and your site stops working.
As a contentious postulate, lets assume that a
Linux based host is 20% more efficient. Buying more hardware to make up this difference is going to cost say an extra 30 pounds per month.
Unless you have an unlimited budget there is also the consideration of development costs for your site. I happen to believe that Dev Studio 2008/VB.Net/ASP.Net2 is the fastest and most efficient development environment. As this makes the site cheaper to develop you recover the increased hardware costs.
So if you have a complex site it may be ASP forces you onto Windows unless you want to try Mono.
Bye
Ian