HaKatan, yontiff is clearly easier to say than yom tov. With both n and t, you touch the tip of the tongue to the alveolar ridge. morewords.com lists only 30 words that have an mt combination, while there are “more than 2000” words with nt (2000 is the point at which the website throws up its hands).
Another advantage of yontiff is that when you see the pope on Yom Tov, you can say “Gut yontiff, pontiff.”