I would go with the cornish rocks too. Their growth rate is amazing but when it comes down to it, it's your prefrence. 
Do you want  a bird that is ready in 6-8 weeks or 10-12 weeks? 
CRB have a little more flavor due to an older age.... Does a little flavor out weigh 4 more weeks of having them? 
In a tractor the CRB's will consume more grass more quickly, They are meant to range *hence the name*. Compared to the Cornish Rocks that are bred for confinement. Broilers do well in tractors.... CRB's do better on an open true free-range style. 
Ask yourself these questions and see what's best for you....