So, Jonathon, are you planning to tell us the correct answer? In your considered opinion, of course.
The traditionally correct form is the one with four syllables, not three. I say
comfortable and
vegetable with three syllables each, but those aren't really analogous to
poinsettia. In those two, schwas are deleted, reducing the number of syllables. It's very uncommon to delete the vowel /i/ in English, as is done in
poinsettia. I'm not positive, but I think the three-syllable version of
poinsettia is a holdover from an old rural American accent where /i/ and /i?/ in final position were reduced to /?/.
And here's the honest truth: it's a pet peeve of mine, but a rather mild one. I'm just a hypocrite like that.