There are several reasons why I came to the conclusion that Waheguru mantra is more superior then Ram.
Baba Nand Singh ji has said, Blessing of Sri Guru Granth Sahib is supreme and He advised everyone to invoke that blessing by completing one path of Sri Guru Granth Sahib in a month in the following manner:
Complete 80 Malas daily of Gur Mantar 'Waheguru'
Complete 160 Malas daily of Ram (Nam) / Or Allah
Now you see it takes less effort to get same fruit by reciting Waheguru .
The mantra waheguru has been mentioned in the Vedas, but noone capable has ever been able to give it till Guru Nanak Dev ji has given this to sikhs as the gurmantra. The whole essence of all the four vedas is that the there is nothing higher then rememberance of God, through God's name. In Sukhmani Sahib it says a lot about the significance of the naam simran, for example you can cut your body into peices and offer it to the lord, but still it is not equal to even one moment of rememberance of Lord's naam.
In the Se Kinehiya Biography, it is written, an ancient hindu rishi Vyas (I believe, reciter of Bhagavad Gita) once visited Baba Harnaam Singh Ji, and said that "we were not blessed with this bliss in our times, this blissful gurbani Guru Nanak Dev ji has given his sikhs, but sikhs are not taking benefit of it"
If you listen to Giani Thakur Singh ji's, Jap Ji Sahib katha tape, he also says, that Ik OngKaar originated from Waheguru (Word/Mantra). Now this knowledge has been passed through Sant Gurbachan Singh ji and previous Jathedars who were all Brahmgianis
Bhai Gurdas ji has said also the composition of Waheguru Mantra, reciting it the Lord of all the four ages is remembered as each syllable corresponds to one of the four ages. Plus it also means the Wonderful Lord and fills us with wonder.
Sant Maskeen has said too from his personal experience that "Waheguru mantra is more effective then other ones like Ram, Hari and it takes lesser effort"
Also I like to quote the following from the introduction in "Vol 2, Translation of Guru Granth Sahib ji" by Dr. Gopal Singh
"It is interesting to recall that not long ago, when Ramakrishna Paramhansa, the modern Hindu Savant, was at his most critical stage of theophanic development, it was a Sikh ascetic Udasi Totapuri, who imparted the Paramhansa, the Sikh esoteric instruction efficacious for removing impediments on the spiritual path, and that is why the most illustrious Chela/disciple of the Paramhansa, Swami Vivekanand, so often uttered and introduced into his writings, the sikh mystic formula, Vaheguru"
I personally also feel its so easy to remember Vaheguru, on the inhale in the mind say/remember Vaahe and on the exhale remember/say Guru mentally.
Waheguru, there is no difference in the Guru and the Lord, one and the same.
Waheguru is the key , the Guru, to open the door of the Lord. When reciting Raam you do remember the Lord, but not as the Guru. In Vaheguru, wonderful Guru, this fills you with wonder, and in total awe of the Guru, your ego vanishes as insignificant.
Now though how can you think of the wonder, when you do not know how wonderful the Guru/God is??
Through the mool mantra or the root mantra, It tells you the description, if you ever have deeply thought over/contemplated deeply over the indivudual words and meanings in the mool mantra, you will be naturally filled with awe. you can have a " mental form" of the formless Lord through the words of the mool mantra. This will become very very subtle and fills with awe and bliss.
In the last, I will tell that by mere recitation of Waheguru or Raam will lead nowhere. Through Guru ji's grace one is blessed to remember the lord with every recitation of Waheguru.