I've used both, and for just getting things done (especially on a budget) I personally prefer MySQL.
I use SQL Server at work, because we are a "Microsoft Shop". I've also used Oracle and MySQL at other jobs.
I also do development "on the side" for small business websites. Sql Server is prohibitively expensive. These folks are NEVER going to have terabytes of data. For "bang for the buck", there is no comparison between MySSQL and SQL Server. If I was doing a large DB with a high number of transactions, I would choose between MySQL with InnoDB or Oracle.