On the help side...
MySQL will work very well with ASP. That is not a problem.
MySQL being faster... not sure with the current build... But I doubt it. MySQL has traditionally been used heavily for web applications, where MSSQL has been used for all manner of DB applications, online and offline.
MYSQL being a real RDBMS... I've heard that it's been upgraded, but dont' really care to look into the specifics at the moment. That could be false. MSSQL is a true RDBMS, which guarantees that your data will be referentially intact if you practise decent design.
I'd go for MSSQL because it is well supported, documented, reliable, and just nice to use. MySQL I find a bit obtuse and I absolutely hate the way it handles dates when used with ASP.
Just my 2 cents.