Welcome to posting on FT. I recommend Hilton. I'd save the points for another time.
Both Westin and Hilton are close to Osaka's main center - Umeda. However, Hilton is better located IMHO. Just across the street from Osaka Station. (Westin is a hike or a shuttle bus ride away).
Hyatt regency Osaka is way off in the docklands. I don't recommend it unless you have some reason to be there.