I have raised and butchered both... Agree that the Cornish X (8 weeks and butcher) need to be placed on a feeding regimen to avoid know issues. I prefer the slow broilers, it gives you an extra month of growth without any issues (in my experience).. I also place them on a feeding regimen, feed during the day only. I also give them over 400 sq ft. of ranging space. They are active and very friendly and at 12 weeks the roos were ~5-7lbs, with the hens ranging in the 3.5 -5lb slot. It really boiles down to to feed to weight ratio's, i.e. cost per pound
I'm more into quality vs quantity, so i do prefer the slower broilers, and if you really don't mind the extra time and feed expense, I would suggest raising a good dual purpose chicken. Such as rocks, marans or orpingtons, the wait is worth the extra flavor.
I hope this helps.
Thanks,
John