Most of the features are the same on both however I recommend Firebird
1. FB - supports multiple collation within a table
2. FB - more choices on mature drivers to work with
3. FB - connection managed by either Thread or Process depend on your use/setup
4. FB - embedded support
Not recommend
Stay away MS SQLExpress
A little Draw back
1. FB - blob handling is not so great if condition below are true
but if you move blob column(s) into separated table, then no issue with blob
a. massive data mining/manipulating
b. blob column(s) is/are defined with other data columns
