Avoid microsoft like the plague. Always push to use PostgreSql even on windows, way better support for developing applications for e.g. Java/Python and still has good support in .NET. Also of course is completely free which given the current license fees for SQL Server is nothing to be sniffed at even if you're a multi-million dollar company.
For the cost of 1 SQL Server license you are going to save £30,000 (say $40,000) or more - buy better hardware to run Postgres on and still have a net benefit.
As far as performance, really if this is such a massive issue we should not be using either DB2, SQL Server or Postgres anyway. The difference between the three is negligible for their design purposes.
Edit: On the .NET integration, actually this is really poor in SQL Server anyway, it does have more features than Postgres/DB2 admittedly but it's not really hugely advantageous over SSIS or stored procedures. I could see the main use case in my work as accessing classes and functions from a CLR .dll but then you're implementing logic in the database which may or may not be a good idea for you.
