If osage sapwood hasn't been compromised in it's curing, by bacteria or mold, it is perfectly fine bow wood. I've seen all sapwood, or sapwood backed osage bows. I have one floor tillered. Black locust has been treated exactly the same way, by convention, for the exact same reason.
If you have access to both, and appetite, I'd do both. But all else equal, cost, quality of the speciment, ect., osage is generally regarded as the preferred choice. This is particularly true for the novice bowyer, where little hinges encountered during the tillering process might be more likely to chrysal with black locust.
