duplicate vs fair isle, yarn stores

[hellebelle.livejournal.com]

i would like to make this scarf. i am undecided whether to do the little alien dudes using duplicate stitching or fair isle. which one is easier/looks better? also, is it a given that scarves knit using intarsia/fair isle/duplicate stitching need to be backed with another knit piece (i.e. to cover the back of the piece)?

also, where's a good place to buy yarn in toronto? lewiscraft doesn't have much of a selection, and lettuce knit is kind of expensive (but the yarn is so pretty!).