Up until now the choices have been rather easy for the most part, but this one is tougher.
Agreed -- I'm having a hard time deciding. They're both strong in their own ways, but neither is really a great one. "The Survivor" has a fairly good character story and, yes, a terrifically imaginative alien design. The story structure has some weaknesses, though, with all the quick knockouts and impostures one after the other coming off as a little repetitive. It's also implausible that neither Spock (who's very observant) or McCoy (who works in sickbay every day) would've overlooked the extra exam table.
As for "One of Our Planets...," it's more of a grand cosmic adventure-type story, but it has some nice character touches with Governor Wesley. It has some fair-sized conceptual problems, though. How did they conclude the cloud was of extragalactic origin? Since when is a nebula the size of three gas giants "immense?" Real nebulae are light-years across. The tech talk about having a separate "antimatter nacelle" is weird and hard to reconcile with subsequent understandings of warp technology. And Spock's ability to do a remote mind-touch, not to mention the computer's ability to translate his thoughts into speech, is a power boost that's hard to believe.
In terms of which one I just plain like
better, I don't think I can choose. But in terms of which one holds up better under analysis, I'm inclined to go with "The Survivor" (assuming we go with ADF's explanation for the deflector issue).
"One Of Our Planets Is Missing"... certainly feels like something that could have been done live-action if TOS had had access to slightly better f/x resources.
Interesting you should say that, because they essentially did just that six years later. Think about it. A vast cosmic cloud demonstrating great destructive power is headed toward a human-occupied planet. The Enterprise
travels into the cloud and discovers it's an intelligent entity. The ship heads toward its brain center, and Kirk orders Scotty to ready the self-destruct. Spock makes mental contact with the entity and discovers it doesn't understand that the humans on the planet are people. Ultimately it's persuaded to go away just in the nick of time.
Where else have we seen that sequence of events? In Star Trek: The Motion Picture
. People always compare TMP to "The Changeling," but I've always found it bears a striking resemblance to this episode instead.