Cornish X are what the commercial chicken operations use. It's what you buy in the grocery store.
They are the best as far as growth rate.
There are other types you can get that many on this site think are better. They don't grow as fast, but have advantages as far as pasture raising.
I think many like the "Freedom Rangers" or something along that line.
For the most part, any bird raised specifically for meat is going to be a hybrid.
And yes, for the meat birds, you generally use both. Cornish X are going to be ready to slaughter before they are sexually mature anyway. A fryer is around 6 weeks old, and a broiler a couple weeks older.