According to us, everything is form of God, but without knowing this concept also people could be serving people. There are many people in the world who does not worship Gods or do not know about such concepts as they have no opportunity. Even though many of them could be serving devotees who actually worship God. How do this scenario is handled in Sanathana Dharma? What difference does this make? Which is considered best, serving devotee or directly serving God?
There is no difference between serving a devotee and serving God. In fact the question need not even come up. Here is why:
Who is God? Where is He present? Sri Krishna says thus in Bhagavad Gita 10:20:
I am the Self, O Gudakesa, seated in the hearts of all creatures. I am the beginning, the middle and the end of all beings.
So He is present in all. So there is no difference between serving one being vs another, whether they are devotees or God Himself, so long as one is aware of this fact that God resides in ALL.
Secondly, what is service? Does God need our service? Sri Krishna says this in Bhagavad Gita 10:41 42
Know that all beautiful, glorious, and mighty creations spring from but a spark of My splendor. But what need is there, Arjuna, for all this detailed knowledge? With a single fragment of Myself I pervade and support this entire universe.
God can do everything. There is nothing He cannot do. Why does He need Our service? Who are we really serving. So there is no question of serving God. All service is merely gift or opportunity for us!
What about serving devotees of the One:
Sri Krishna says this in Gita 9:22 verse of assurance:
But those who worship Me with devotion, meditating on My transcendental form--to them I carry what they lack and preserve what they have.
So Lord Himself takes care of all needs of His devotees, so what is the meaning of us serving them. This question too does not arise. We are serving no-one nor is there a need to serve His devotees, for God Himself takes care of Him.
Finally the last point, which is 'better' service of the two. This means that there is a result for service. If the result of X is better than the result of Y, then X > Y right? But Sri Krishna says this in His Bhagavad Gita 2:47
Thy right is to work only, but never with its fruits; let not the fruits of actions be thy motive, nor let thy attachment be to inaction
So God says that there should not be any attachment or desire for rewards when performing actions. Such duty is liberating and therefore becomes best. So then which produces better fruits: serving devotee or serving God? Neither, for the best is to serve WITHOUT thinking of rewards/fruits, in other words, just go ahead and SERVE, do not think which is better.
SO there is no difference. Go ahead and serve all beings, fixing your mind on God (all forms are nothing but His Forms - God is One) and without expecting any fruits in return for such service, because after all there is NO SUCH THING as service, because God does not need it, nor do the devotees. All the best sir!
This is just one possible interpretation from scriptures, and there may be many possible ones.