We buy antlers for our dogs, which they love. Since your dogs are voracious chewers, don't buy the split anters, b/c they won't last long enough; buy the whole ones. They're expensive, but they last longer than Nylabones and are more natural. If you want to stay on the Nylabone path, I'd buy the next size up.
I quit giving my dogs marrow bones years ago when my lab, Chipper (RIP!), broke a canine tooth on one. He was quite the chewer, too. We switched over to Nylabones, buying him a new one every week or two.