I will start by saying I am no expert on this. :)
I believe the hex. is the taller artabotrys also called climbing ylang ylang. I have seen one about 15' tall here. Someone told me the siamensis is the same as the odor. but I am not sure. I know the siamensis/odor.(?) smells wonderfully fruity and the hex. smells closer to the Cananga flower IMHO.

I have artabotry plant, which i have bought last year on ebay for $29.99. about one and half feet tall. i live in zone 7b/8. I keep the plant inside all winter. this summer i planted it outside. Today i was pulling the weeds around my plants. I have noticed nice smell. then i realized there should be some flower. i have searched for the flower, i have seen a flower and another is comming up. I am so exicted to see the flower. My plant is not really big. its in 5 gallon pot. its blooming. I am so happy to see flowers.

Artabtrys hexapetalus, odoratissimus, siamensis. Trying to reserch the differences is difficult. In certain parts of the world it's common name is "false ylang ylang" The scent of the flower is described as fruity, mixture of banana and lemon. Plant attraction nusery states that the odoratissimus scent is the closest to Cananga odorata(giant tree). So I guess if our yards are too small to grow the large Cananga odorata, we must settle for one of the three ylang ylang vines. I puchased a rare dwarf ylang ylang, 21 in. high for $ 60. It's a pretty plant but the flower's scent is very mild. It's mature height is said to be 8 - 10 ft. With the abundance of flowers the scent might become worthwhile for the high cost of the plant. Good lick to all you Yland ylang lovers. Aloha from Hawaii.