Microsoft provides a comparison between MySQL, SQL Server Express, and SQL Server Enterprise. Just go to http://www.microsoft.com/sqlserver/en/us/product-info/competitor-compare.aspx and pick MySQL in the dropdown box. Take it with a grain of salt, as the information is provided by Microsoft, the maker of SQL Server.
If you go to http://www.microsoft.com/sqlserver/en/us/product-info/compare.aspx and click on SQL Server Express, you will see just how restrictive the limitations are:
Express will run only using one CPU, will only use up to 1 GB of RAM, and will allow you to have a database up to 10 GB only.
If you do not want to face all those restrictions you should go with MySQL. MySQL provides InnoDB engine which is fully ACID compliant, you can have online backups by using mysqldump with correct options with InnoDB tables or by using Xtrabackup from Percona. I would recommend going with Linux for MySQL as it is usually a more stable platform and gives you more flexibility since many tools for MySQL are written for Linux. MySQL has replication and high availability if you go with MySQL Cluster.