Brian Swan has posted the second part of his series looking at the difference between the MSSQL driver and the SQLSRV drivers for connecting to SQL Server databases from PHP (part one can be found here). He gets a bit more detailed in this second post.

I’m aiming to provide a high-level comparison that you might use if you were considering moving to the SQLSRV extension, but I think there is also some interesting information if you are just curious about the differences. [...] In cases where a short note wasn’t enough (and there were several of these), I’ve provided relevant links to topics in the SQLSRV documentation.

He talks about things that one offers that the other doesn't, like prepared statements (sqlsrv) and working with stored procedures (mssql). He also mentions error handlng, persistent connections and scrollable cursors in sqlsrv. Finally he gets into a one-to-one function comparison of the two drivers with the sqlsrv function list coming up a bit short compared to the features of the more recend (and well-supported) mssql driver.