I'm lead developer for a company that writes custom software for the mining industry. We
support MSSQL and MySQL. I've found that from a programming aspect (VB + ADO) there is
relatively little difference between MSSQL and MySQL. There is some sight syntax
differences and MySQL versions < 5.0 do not support stored procedures. We use the
InnoDB table type for MySQL as it provides row level locking and transactions. Our
largest client has about 1 gig of data and averages 125 users. I've found that MySQL
usually out performs MSSQL if you tune it properly and use good programming techniques.
It is less integrated with Microsoft products though so if your clients will be accessing
the data via MS Office applications then MSSQL will seem easier. We offer both platforms
mainly because a lot of IT managers are convinced that Microsoft solutions are the best
even when benchmarks say different.