Like a lot of things, I think it can depend on the way circumstances conspire to create a particular experience on any given day. For example, I've seen IllumiNations scores of times - some have been outstanding moments that will endure for a lifetime, others less so.
I've done MVMCP and MNSSHP once each. When we did MVMCP back in 2001, I thoroughly enjoyed it. When we attended MNSSHP with the same group of family and friends in 2002, I came away feeling it was just an excuse to extract more money for the privilege of being in a (very crowded) Magic Kingdom during the evening. It didn't help that our adult friends were determined to stand in every 'Trick or Treat' line, no matter how long, to make sure they got their money's worth of penny sweets (that they were never going to eat anyway)

. Even the kids weren't bothered after the first couple. As the parade ended (at around 10.20pm, from memory) and we strolled into Tomorrowland, the skies opened and we had the most rain we've ever seen in all our visits there. We sheltered under the canopy of The Lunching Pad and waited. And waited. And waited. All the time the rain just got worse and worse. Eventually, at midnight, Matt ran across to the nearest shop and bought umbrellas (all the ponchos were gone). We were all wearing brand new trainers, so we took them off and stumbled, barefoot and ankle deep in water (literally), to the exit. By the time we got to the car at the TTC, we were all soaked to the bone (we soon gave up with the umbrellas). No wonder that I don't have very good memories of MNSSHP! Oh, and we almost got wiped out by the Headless Horseman as he swooped around the bend (that little gem has just come back to me).
I don't think I'd rush back to either, but neither would I rule them out. I do think everyone should experience both at least once.