I prefer the All-In-One training halter bitless bridle. I have used Dr Cook's and it pinches under the chin and the chin straps don't release so my horse ends up shaking her head. Its like a vice grip from poll to chin. I bought an all-in-one and love it. All my horses have them now and our entire barn too. Missy Wryn makes them and teaches how to train a horse bitless www.MissyWryn.com.
Hope this helps