A Rettungsassistent is a rescue assistant, and a Rettungssanitäter is a rescue paramedic. Judging by the names I'd say a Rettungssanitäter receives less training or has a different base of operations then a Rettungsassistent. Again just a guess, and I'd also say that the Rettungsassistent responds with the Artzuen, or doctor. Still I'm shooting in the dark.