Many of the XML api calls take one or the other and sometimes return both. They seem to always be the same and work fine interchangeably. Are there scenarios where SessionKey would be something other than MeetingKey?
These would just be different terms for the same value. Generally with meetings it's referred to as the meeting number/key while with trainings they're typically called sessions (session numbers/keys). In any case, yes you can use them interchangeably.