Consider your draw length. Osage is a very heavy wood and makes powerful fairly short bows for short draw lengths. Yew is very light wood and works well in very long bows for long draws. If you are going to make a 6 foot plus long bow for a long draw, I'd say yew would be the better choice. If you want a 5 or 5 and half foot bow for a short draw, I would say osage.
Yew is very soft and easily worked (and damaged) if you get a straight piece without too many irrerularities. Osage is tough and long lasting but you will need really sharp tools. Either way I would pay extra to get a really straight stave or pair of billets for your first effort. And I would probably back either one with rawhide or sinew as it is easier than a bamboo backing and might well perform better. LBG