While I strongly prefer the term "spawn rate" in the forums to distinguish between the new and old rates, I am with Proxima here - at least for now. There are two reasons I see:
1) The old "Release Rate" and the new "Spawn Rate" are extremely similar, but players new to NeoLemmix already recognize the term "Release Rate", so they know what to expect. On the other hand, "Spawn Rate" sounds like something new that requires (at least a bit of) learning.
2) "Spawn Rate" is rather similar (language-wise) to "Spawn Interval" (which is already called this way in the preview screen, if you use it), however there is a far larger difference between them than between "Spawn Rate" and "Release Rate" (physics-wise). So distinguishing the two ways of displaying this physics already by their first word seems a sensible thing to do.
I am not saying that we should keep "Release Rate" forever, but I would like to keep that word at least for another year. Let's see what NeoLemmix is by then, and then look at this question again.