The descriptions for both of them basically say the same thing. They go out to explore and survey, and they both can be manufactured by the Mothership.
Spoiler
derelict_probe,CUSTOM,"An example of one of the countless thousands of robot probe ships sent out into the galaxy by the Domain to find new resources to exploit, planets to colonize, and explore the unknown. Some variants are produced by automated motherships. This specimen appears to be inactive; pitted by small impacts, its hull displays iridescence from fearsome radiation scarring. This probe's manufacture could well date to a thousand cycles ago."
derelict_surveyship,CUSTOM,"An automated survey ship from a long-past wave of Domain exploration. Thousands of ships like this were sent in waves, some independent, others attached to (or manufactured by) similarly automated motherships following the first wave of robot probes. This apparently unresponsive hulk has been ravaged by centuries of exposure to charged particles and high velocity impacts from interstellar dust."
derelict_mothership,CUSTOM,"One of the legendary survey motherships, the quiet automated workhorse of the Domain exploration and colonization effort. This is in essence a von Neumann probe, as the survey motherships contain autoforge capacity to manufacture a suite of smaller survey ships and the supplies required to perform its ongoing mission. This specimen is burned and battered by its centuries of service to the Domain, a drifting forgotten hulk."
Since the Probe and Survey Ship seem to differ only in size, why would the Domain bother making the larger Survey Ship if a smaller Probe can do the job?
I know I might be taking it too seriously, trying to find meaning in something probably irrelevant. But hypothetically, why would the Domain bother?