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.

Wendell
Pediatrician in Chicago

The patient's expectation is that you have all the answers, sometimes they just don't like the answer you have for them