24. Why are there different mantra-s for different people?
One gets fruits (of actions) in proportion to the strength of bhakti (devotion) and samarpaNa (surrender). What matters is the intention behind chanting God's name.
Each mantra is a vibration (as sound has vibrations). Our subtle bodies, chakras and our mind, chitta, etc also has vibrations. We have a definite personality.
When one's basic nature tunes with the character of any form of Lord, one finds it easy to surrender to him and chant his name. The mantra is so formulated that it’s vibrations will also tune into the vibrations of subtle bodies and chakras and cleanse them.
Just like we have a liking for some food and dislike of other food does not mean that what you like is the best food and rest is not. We can say that certain type of food is suitable to us. In the same way certain vibrations attract you. Have you noticed that when you are standing near a stranger, at times you feel that you are not comfortable in his/ her presence. Sometimes, we become friends in the very first meeting. It’s all due to vibrations. If the vibrations are not of similar frequency, then you might not feel comfortable with him / her. Your thoughts might not align with him / her.
In the same way, a certain mantra is more effective on one person and lesser effect on another.
OM mamo bhagavate vAsudevAya is considered as bIja (beej) mantra (corrections welcome) and so it is powerful than a purANika mantra like ShrI kRuShNa sharaNam mama. But some get more joy in chanting krishna's mantra than vasudeva's. It is because the vibrations of these mantra tune with our vibrations of our subtle bodies and chakra-s
In the same way, some experience great joy in chanting 'ShrI rAma, jaya rAma jaya jaya rAma' while 'OM ShrI rAma chandrAya namaH' is considered as more potent as it corrects one with heart. It is bija mantra.
OM increases potency of any mantra and hence most mantra-s have OM in the beginning. Some also prefer to close or end any mantra by chanting OM, but kRShNa’s mantra (ShrI kRuShNa sharaNam mama) does not have OM in it (atleast not the present version, which is chanted by most).
So it all depends what suits you.
But bIja mantras are technically more potent than other mantras or puranic mantras.