A .dtsx file contains the source code for the SSIS package (in XML format), while .ispac is a ZIP formatted file containing .dtsx files, .params file, .manifest file, and other project files required for a deployment. You can actually open up a .ispac file using a tool like 7Zip and examine or extract what's inside.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho