From the notes from version 9.1
"SQL Express 64-bit is now supported: For new installs - if the user is running a 64 bit version of Windows, SQL Express 2012 x64 will be installed. For upgrades - if the user currently has SQL Express 2012 x32 then the install will keep that version installed. (AC-7943)"
I have not been able to verify this since I have not been able to complete a clean install from scratch on 9.1 without a key over the weekend.
