I got my answer here:
LocalDB and SQL Server Compact?
Small and simple database, lightweight
installation, connecting to a database file -- this will sound
familiar to any developer using SQL Server Compact. The similarities
are not accidental, as our goal for LocalDB was to be as easy to use
as SQL Server Compact (while being as powerful and compatible with
full SQL Server as SQL Express).
There are significant differences between LocalDB and SQL Server
Execution mode: SQL Server Compact is an in-proc DLL, while LocalDB
runs as a separate process.
Disk usage: all SQL Server Compact
binaries amount to some 4MBs, while LocalDB installation takes 140MBs.
Features: SQL Server Compact offers core RDBMS functionality like
querying, while LocalDB provides a much richer set of features,
including Stored Procedures, Geometry and Geography data types, etc.