What is the difference between reciting "Namo Amitabha Buddha" and "Amitabha Buddha"?

Namo means taking refuge, expressing sincere reliance and submission, and Amitābha is the name of the Buddha, the principal deity. Reciting the Buddha's name is done in order to resonate with the Buddha, to be in accord with the Buddha, to seek the Buddha's blessing, and to ask Him to extend His hand to rescue, save, and deliver us. The two methods of reciting the Buddha's name are inherently no different, but with two extra characters, the mind's recitation becomes winding and indirect; it is not as direct as reciting "Amitābha," which keeps the mind simple and makes it easier to attain meditative absorption (samādhi).

Reciting the Buddha's name is not purely for the sake of reciting the name itself; rather, it is to gather and focus the mind, to sincerely take refuge in Amitābha, and to attain the samādhi of reciting the Buddha's name (buddhānusmṛti-samādhi). Only within the samādhi of Buddha-name recitation can one resonate with Amitābha and be in accord with Him, and only then can one have assurance of rebirth in the land of bliss (sukhāvatī). Therefore, in reciting the Buddha's name one should also try one's best to settle the mind; the mind must be single-pointed. When reciting the name, the Buddha should be present in the mind, reciting with a heart of reverence. In order to gather the mind and attain concentration, the voice when reciting the name should not be too raised, for this exhausts one's vital energy and the mind easily becomes scattered and difficult to collect. With a lower, more gathered voice, the vital energy converges, the mind and spirit become tranquil, the mind focuses without distracting thoughts, and it becomes easy to attain concentration, attain the samādhi of Buddha-name recitation, and receive the Buddha's blessing.

In order to keep the mind on the Buddha and attain the samādhi of Buddha-name recitation, one may try reciting in this way: slowly sound out the syllable "A," drawing out the sound of "A" at length, without reciting "Mituo Fo" (Amitābha), while at the same time holding the principal deity Amitābha in the mind. The syllable "A" is revealed outwardly, while the three characters "Mituo Fo" are hidden deep in the heart—one syllable standing in for the entire Buddha, the mind free of distracting thoughts, filled entirely with the Buddha. Reciting in this way makes it easier to attain samādhi.