This is the standard "it depends" answer.
Both of those breeds typically don't reach "maturity" until after a year, depending on strain, it could be two years!
Are you looking for the point where the next pound or two will be ALOT more feed and time, than the last couple pounds?
My suggestion is to think of a dressed weight you want ... weigh your birds, take the live weight, and figure on 2/3's will be dressed weight* (6Lb live = 4 Lb dressed) than butcher one of each breed, and try them ...
*I'm leaving my calculations a little wiggle room ... some people consider neck and giblets in with the weight, others don't, some even include the feet!