MS-SQL.
mysql does not support stored procedures, no functions, no functional indexes, no cascading delete on foreign-key constrains, no subqueries, and a shitty GROUP BY implemention.
Besides which, if you're stuck with MS platforms (a very bad choice for internet-based applications), nothing runs better than MS products (MS make sure of that)
A dead cheap, but very reliable alternative is a linux box with postgresql. more reliable than MS (doesn't need a reboot every two weeks) and is probably a lot faster too.
If you're bent on spending cash, get a good size SUN machine with Oracle.
Remember, the database does not have to run on the same OS that the applications run on. If it's a heavy app, you're going to need a seperate server anyway, so you might aswell make the best choice for the database (which is not MS-SQL on windows)
