Those are dual purpose breeds so essentially no difference because of breed. There is no "best" age for everyone. There are different things to consider.
The age you butcher can affect how you cook them. As they get older and as the hormones hit those boys in adolescence the meat gets more flavorful and develops more texture. I don't like giving specific ages because we all have different tastes and tolerances plus different cockerels hit puberty at different ages. Most people are OK with frying or grilling a 12 week old, some are OK with 14 or even 16 week olds. Then there is a window where they can be roasted. The age limits on that might depend on your technique. But in general the older they are the slower and moister they need to cook. Owe glaring exception to that is pressure cooking, that is moist but it is also hot and fast. My target age is 23 weeks and I usually bake them in a sealed pan in the oven at 250 F for about 3 hours. You can also use a crock pot or make stew or soup with any of them,, young or old. These methods are a way to turn even a tough old rooster into a good meal. They never get too old to cook but you do have to change your methods.
Some people like to butcher cockerels pretty young, maybe as young as 12 weeks to try to get them before they start to crow if they are not allowed to have roosters. Some try to butcher them pretty young before puberty hits and they start to show an interest in the girls or fighting among themselves. So behaviors can play a part.
The younger they are the less meat there is on them. At 12 weeks there is very little meat, they are mostly bone. But you can still fry or grill them and that's before most behavioral issues show up. At 16 weeks most seem to hit a fairly nice size. I choose 23 weeks because that's about when their teenage growth spurt seems to really slow down and the way I like to cook them. And I'm often looking at behaviors and growth to see which I might want to keep for a replacement for the top rooster, which will be eaten, by the way. They are never too old to eat.
How you feed them might enter into your decision. If you are buying all the food they eat you might want to go kind of early to reduce that expense. If they forage for most of what they eat then it should not cost you that much to let them grow.
Lots of variables. The best age is different for each of us because of our desires and conditions. I have no idea which is best for you. But whichever way you go I suggest you consider making broth. When I butcher I cut the bird into serving pieces for the table and save the neck, back, and other pieces specifically for broth. I also save the bones when I cook the other pieces and use them. If you cook the whole carcass use that after it becomes leftovers. Part of that is that the broth is really really good. Part of it to me is that it seems more respectful of the bird if you use more of it. It's life wasn't a waste. But that's just me.
