IMA teaching is more detailed and has 5(?) drills to ingrained the feeling of a correct IMA swing.
There are a number of drills (more than 5 in total) but I don't know that these are more detailed than the BGG drills for example.
Pros: more power and accuracy.
IMA allows for a longer backswing. This does not necessarily result in more distance however those that have the fault of not holding the angle long enough for a proper impact position (more than 90% of ams in my examinations) make up for the distance loss with a longer swing and so many loose distance with short backswing swings. Fix the fault and the distance will be roughly equivalent.
Cons: need dedication to the drills.
True of all of these swings.
Good swing option for the better athlete?
Athleticism not required.
Graves brothers is an off shoot of Natural Golf.
All of these are. Scott Hazledine (IMA) was the first director of instruction for NG. Tom Sanders (BGG) was a top NG instructor.
Pros: worked closely with M. Norman.
If you want to reproduce Moe's swing exactly this is a pro. Some have said that there are essential parts of Moe's swing and then there are the idiosyncratic bits. At a macro level Todd's swing looks a lot like Moe's. When you look at some fundamentals they were not nearly so similar.
Cons: not as detailed as IMA.
There are FAR more Graves videos than IMA
How much difference from Natural Golf?
It depends on which Natural Golf you mean. There have been a number of very different NGs over the NG taught by JK to the one taught by Scott to... Todd was the developer of NG instruction at some point(s) so there could be no difference or a lot depending.
Natural Golf started the Single axis revolution (marketing wise?) Pros: Been around, possible instructors nearby with a background in teaching NG. Cons: Limitations in how far you can improve?
I don't think there is a limitation on how far you can improve with NG. The 'Pro' of a nearby instructor can be a 'Con' depending on the local instructor. Chaz has some interesting stories about the differences between local NG instruction in the past
Some NG instructors are not NG users - They are CG teaching pros that learned NG to expand their market for instruction. While this may not always be a negative it was in the case of the one such that I've observed.