Rob,
Osage is sexy and just LOOKS like bow wood. If this is your first attempt at building a bow, use the hickory. It does have problems with soaking up water out of the air, but that can be managed easily.
I have a 31" draw length and have a hickory bow that is one of my favorites. Unless you just WANT to build a beast of a bow, backing it with deer raw-hide will give you better lines and a narrower bow. I wouldn't shift off of a 68" to 72" length. Go with a board bow build. Easier to find materials and get cut out well.
Hickory is not as "snappy" as Osage. It might feel a little doggy in your hand when you shoot it, but a tougher bow that can also be used as a quarterstaff, you will not find.
Watch the area where the fades translate into the limbs, it's easy to develop a hinge there.
Have fun! If you screw it up, slab it out and make knife handles out of it.
Holler if you would like to see pictures of my hickory bow.
OkKeith