Definitely go with something else that is faster to mature. Look for a breeder to obtain stock as hatchery birds are not bred for the table rather for egg laying resulting in small, thin, poor table birds.
Cornish are indeed a great choice but they don't lay many eggs with majority laid Spring. Which works as you'd hatch and grow all for table birds and few breeders. New Hampshire, Buckeye or Wyandotte are better choices for dual purpose birds than Plymouth Rock. I raise Plymouth Rock.
Best age for processing dual purpose birds for broilers is 12 to 14 weeks. It's a good time to cull out a bunch of cockerels and grill them up. This will save on your feed bill as the little gain of meat from this age on is at 1/4-1/3 lbs feed per day per bird. Fryer age is up to 18 weeks old and the coveted (here on BYC) Roasting age is best around 24 weeks.
Note the age is related to cooking method. Older birds can not be broiled. Must use a slower, moist cooking method the older a bird is.