I have to go with the Eastern. They can be down right tough to hunt. I've never hunted the Osceola, but from everything I hear, they can be just as tough to hunt as the Eastern. IMO, Rio's are a whole lot easier to kill than Easterns. I hear Merriams are even easier than Rio's. I do love going to south Texas to hunt Rio's, but as for a challenging hunt, it's hard to beat the Eastern.