I had a similar question not that long ago. I spend a lot of time looking at youtube videos and websites for learning guitar. I discovered marty a while back, and enjoy his videos some. I saw DT the other day and enjoy those too.
It was looking at the DT videos that I realized the value of the Justin courses. I was getting all excited about "making things musical", and thinking "this is way more fun than the Justin stuff". Then it hit me. None of that would have been any fun if I didn't already have the structure and understanding that I got from what I have done in this course.
Marty and David both like to describe how to play songs by saying "ok, for this is the pattern for the song, play down down up". That's the way a lot of people do it too, but what does that mean? Where does that fall in the pattern? Is it 1 3&? Is it 1, ,2, , , &? Or is it 16ths and it's just 1 &a?
They don't tell you these little things that really enable you to start breaking out of learning songs from examples, and into figuring them out for yourself. It may seem trivial to some, but things like that really put me off some lessons. It makes it seem more like mimicry than learning, and I want to learn.
I have found a few of the things DT has in his videos useful. I like his finger independence exercise better than Justin's (almost the same thing, but DT is harder). I also like his idea of stringing chains of chords you are working on together and practicing them rhythmically, so I've added that to my practice too. I'm sure I would find similar things with Marty.
I've not bought any of them, and I'm sure they all do a great job with what they do, but if I were to choose one to purchase, I would have to go with Justin just because of the broad scope of material he's covered. I just don't see that sort of depth in the other two. Their courses may seem more "fun", but I have a feeling that there would still be a lot of remedial theory and aural work to do afterwards.