Lots of things can contribute to the flavor of the meat. But in your case, it's probably the age difference. Cornish cross are processed young, before they are even 3 months old. They may be big and meaty at that age, but they are still physically babies. A 5-6 month old dual purpose bird has had almost twice as long to mature and most of them will have started to have at least some hormonal changes, which definitely changes the flavor of the meat. Delawares are also far more active than a cornish cross is going to be, which means that they will use their muscles more, resulting in more blood flow to those muscles, which also contributes to how the muscles taste when cooked. The differences are kind of like eating veal vs beef. If your cornish were penned and the delawares were free ranged, then I'm sure there are also a great number of differences in the diets between the two and diet certainly can affect the flavor of the meat just as it can affect the flavor and texture of eggs. Think about the differences between store bought eggs and your fresh from the nest box eggs...some of those differences are due to freshness but most of them are a matter of different diets. If you caged your hens and fed them only layer feed your homegrown eggs would soon start to resemble store bought commercial eggs because you would be raising them the same way the commercial layers are raised.
It's nothing you did wrong, it's just the difference between the breeds and the ages of the birds at butcher time. Thanks for posting though, we have a bunch of cockerels to process this weekend that will be 17 weeks old. These are the first birds we've butchered that haven't been mature roosters or spent hens and I was starting to panic that they might be getting "too old" to use faster cooking methods like frying or grilling. Knowing that you had some that were a month older that were still tender is very comforting reassurance that we haven't waited too long in the interest of letting them get just a little bigger.
