Just FWIW, but MSEP had runs of 14, 2 and 6 years at MK (so ~22 years total). Spectro had runs of 8 and 9 years (so ~17 years total).
I know that on these pages there is definitely a preference for Spectromagic, but I'm not convinced that "typical" guests of the parks who have visited WDW would consider it more memorable than MSEP. Also, because of the MSEP runs at different parks around the world, I suspect that a lot more people associate it and Baroque Hoedown as a classic "Disney thing" than On This Magic Night.