My immediate response was going to be, "Whichever you can do faster." For me, that would be crochet, esp. something like double crochet (stitches).
I agree with the above advice: learn both skills first. Decide which one is more comfortable for you GIVEN that you want to make a completed scarf by Christmas. Both skills may be comfortable; what a problem! ha ha
So then it will be a matter of which look you prefer and the speed at which you can work. My crochet is MUCH faster than my knitting; I've known how to crochet for over 20 years, but I only learned to knit in May 2011.
Good luck, and come back with questions. Yarn stores (even Joann's and Michael's) offer classes in basic knitting and crochet, so don't think that the only way to learn is via on-line videos. There are also books--I learned how to knit from a book aimed at teenagers, so don't discount anything!