Your Body position in a Good-Morning should resemble a conventional Deadlift more than a Romanian Deadlift.
Good-Mornings also prevent you from using weight that’s too heavy. If you Deadlift 405, you’ll probably be stuck doing Good-Mornings around 225. But you can probably do RDLS with 315 or more. The Good-Mornings will be more difficult, but the RDLs will put a greater load on your body.
I also think that RDLs just hit the Hamstrings more while Good-Mornings hit the Glutes more. I can’t really explain why but it’s just what I experience. If you’re a Hamstring dominant person like a sprinter, you might want to choose Good-Mornings over RDLs. If you’re a bodybuilder and you want your Hamstrings to hang but you want a tiny waist and a tiny butt, then RDLs are probably better. It would be good to use both.
Good-Mornings are about as close to a Glute-Isolation exercise you can get and RDLs are the only Hamstring-Isolation work you can get at most gyms without using a Leg Curl Machine. [/quote]
Good Mornings should not look like a conventional deadlift. In a conventional deadlift your knees should go forward, equal to or often beyond your toes. That should not happen at all in a GM. In fact, a GM should look a lot like a… RDL.
I agree it is possible you might feel the RDL more in the hams, but to say a GM is a glute isolation exercise is not accurate. The erectors and hamstrings play a huge role in the exercise as well. Nor are RDL’s a hamstring isolation exercise given the glutes, erectors, traps, and forearm flexors are all heavily involved.