SQL Server Express is a more complete solution for a database backend. It's more fast, more robust, etc.
If you are considering Express version it's because you're not thinking in massive amounts of data because Express version has a limitation of 2GB. If you are confortable in working with Access 2007, my sugestion is that you stay with that.
If you're thinking in changing in the future your frontend for example to VB .Net, you should change already the backend to SQL Server Express because you will be prepared for the future.
jppinto