SwamiX,
The I AM mantra has specific effects that work with the AYP system and should be used instead of the TM mantra if you are practicing AYP. There are many articles here explaining why.
And there are subtle differences between DM and TM.
This is purely speculation, but part of me feels like there is an advantage to the universality and open-source transmission of AYP’s set of mantras. Even though the process of refining mantras is deeply personal and happens in the interior of the neurobiology, we all know that the effects of cultivating stillness ripple out far beyond an individual person’s body.
Therefore, it would seem that the more a particular vibration is utilized and effectively sewn into personal and collective consciousness, the more power that particular vibration will have moving forward. So, the more a particular mantra is used, albeit internally, the more traction that mantra will acquire on a collective level. The same could possibly be said about a specific set of sutras in samyama (which have a more outward direction from the get-go).
Just contemplating some possibilities here. 
Another possibility is that different mantras open and stimulate different energetic pathways. We all know about the transformative power of sound. So by utilizing multiple pathways it would make sense that multiple doors are being opened to the nervous system chakras etc.
I agree with this hypothesis, that the subtle differences between the ™ mantra (at least my ™ mantra) and the DM mantra are significant. I can’t really evaluate the differences between mantras except at an experiential level, and Ive only used these two.
I had trouble adjusting to AYAM for several months and I still find the other mantra coming back from time to time. I don’t try to influence it, except still to gently return to AYAM when I notice I have drifted off of it. I strongly believe that meditations of these types (perhaps all meditations?) are intended to be passive except for the most gentle application of control.
But that means to me, that actively selecting and controlling which mantra to use and perhaps the faithfulness of its internal utterance are all “too much control.” Mantras evolve within a given sitting. It can become finer and thinner and longer, or resonate with the power of a gong. These things are mine to experience, not mine to control, in my opinion.
The tantras has that Hreem has both male and female properties. Shiva (hr) Shakti (ee) Union (m)
Dr. David Frawley and Pillai also elaborate on this.