I don't think 5 is better than 6, but I ddefinately think it is underrated. Liek you said, not every episode has to have emotional depth, it can just be a fun episode to watch. In fact, I love last Temptation, I found, surprisingly, it beats out Colonel Homer(#11 out of my top ten) and makes it in around number 4 I think on my top 25. I don't mean any offense to Matt Groening, but I just feel it is a slightly better episode(though almost all, if not all, the episodes in my top 25 are A+'s, so they are still really close)
On another note, Just watched One Fish, Two Fish, Blowfish, Blue Fish again, and am I the only one who gets very close to tears everytime I get to the end of this episode. This must be the 10th time atleast I have seen it, and every time my eyes whater(though I don't shed a tear) This is definately up in my top three tear jerkers, even though it didn't exactly get me to cry. Anyone else do this when they see this episode?