Reward is directly related to the efforts. What you put in will determine what you will get out.

Class A Devotee

gosthy-anandis- Engaging others in devotional service

Class B Devotee

Bhajananandis – Devotion for personal benefit

1 This devotee spends his time on preaching by giving 100 people the Bhagavad-Gita book or some teachings from the Bhagavad-Gita. This devotee chants mantras, engages in Deity service, and performs austerities. No preaching.
2 Maybe 30 people will be read. A few people will be inspired upon coming across or hearing about this devotee and will also carry out the devotional activities mentioned in 1.
3 Maybe 3 people will learn something.
4 Maybe 2 people will implement a BG teaching into their lives.
5 1 person will spread the knowledge to others. The whole cycle of 1 to 5 will be repeated.
Result of Devotee Efforts Thousands of people will benefit from the Bhagavad-Gita teachings and hundreds of sincere devotees will be created. Very few people will benefit due to the efforts of this class of devotee. A few may become sincere devotees.
Reward for Devotee Residency on the same planet as Lord Krishna. Residency on one of the millions of spiritual planets.

Without preaching, even the most righteous religion would become extinct. With preaching, even the most evil religion can become dominant in the world. This is the reality. God is not going to spread righteousness nor evil, it is up to us. Lord Krishna has given us the most righteous and most advanced spiritual knowledge in the Bhagavad-Gita. It is up to us, whether we spread it or not.

“Prahlada Maharaja, a pure Vaiṣṇava, prays to the Lord not only for himself but for all other suffering living entities. There are two classes of Vaisnavas — the bhajanānandīs and goṣṭhy-ānandīs. The bhajanānandīs worship the Lord only for their own personal benefit, but the goṣṭhy-ānandīs try to elevate all others to Kṛṣṇa consciousness so that they may be saved.” (Purport by Swami Srila Prabhupada, Shrimad-Bhagavatam 7.9.41)

“…One should become a guru and spread Kṛṣṇa consciousness all over the world, simply by preaching the teachings of Bhagavad-gīta.” (Purport by Swami Srila Prabhupada, Shrimad-Bhagavatam 7.9.41)

“The more we are dedicated to the deliverance of living beings from this rotten material existence, the more our own position of liberation becomes assured. This is why we see that the most advanced devotees are those who are actively engaged as much as possible in giving Krishna consciousness to others.” (Swami Srila Prabhupada)

“For one who explains this supreme secret to the devotees, pure devotional service is guaranteed, and at the end he will come back to Me. There is no servant in this world more dear to Me than he, nor will there ever be one more dear.” (Lord Krishna, Bhagavad-Gita 18.68-69)

“One who liberally disseminates this knowledge among My devotees is the bestower of the Absolute Truth, and to him I give My very own self.” (Lord Krishna, Shrimad-Bhagavatam 11.29.26)

Preaching is More Important than Deity Service

“Deity worship is especially meant for purifying the neophyte devotees. Actually, however, preaching is more important. In Bhagavad-gītā (18.69) it is said, na ca tasmān manuṣyeṣu kaścin me priya-kṛttamaḥ: if one wants to be recognized by the Supreme Personality of Godhead, he must preach the glories of the Lord. One who worships the Deity must therefore be extremely respectful to preachers; otherwise simply worshiping the Deity will keep one in the lower stage of devotion.” (Purport by Swami Srila Prabhupada, Shrimad-Bhagavatam 7.14.40)

Preachers are Worship able

“My dear King Yudhiṣṭhira, the brāhmaṇas, especially those engaged in preaching the glories of the Lord throughout the entire world, are recognized and worshiped by the Supreme Personality of Godhead, who is the heart and soul of all creation. The brāhmaṇas, by their preaching, sanctify the three worlds with the dust of their lotus feet, and thus they are worshipable even for Kṛṣṇa.” (Shrimad-Bhagavatam 7.14.42)

Lord Krishna is most pleased with those who preach and spread the Vedic knowledge.