Hi difference in Lord Vishnu and Lord Krishna.
There is no difference between Shree Krishna and Maha Vishnu. They are one and the same.
This fact is clearly stated in Shreemad Bhagvat Purana Skandha 10 -Chapter 89 -Shloka 58 .- -Kṛṣṇa and Arjuna Retrieve a Brāhmaṇa’s Sons.
This is an interesting story about a Brahmana who's dead sons are brought back by Shree Krishna and Arjuna , when they visited Lord Maha Vishnu's abode in Kshira Sagara.
Shree Krishna offered homage to Maha Vishnu i.e. he himself. Then Maha Vishnu told them that he himself brought both krishna and Arjuna here so that he could see them both. And they are the Nara-Narayana Rishis .He also adviced them to follow Dharma.
ववन्द आत्मानमनन्तमच्युतो ।
तावाह भूमा परमेष्ठीनां प्रभु
र्बद्धान्जली सस्मितमूर्यया गिरा।।58।।
vavanda ātmānam anantam acyuto
jiṣṇuś ca tad-darśana-jāta-sādhvasaḥ
tāv āha bhūmā parameṣṭhināḿ prabhur
beddhāñjalī sa-smitam ūrjayā girā
Lord Krsna offered homage to Himself in this boundless form, and Arjuna, astonished at the sight of Lord Maha-Visnu, bowed down as well. Then, as the two of them stood before Him with joined palms, the almighty Maha-Visnu, supreme master of all rulers of the universe, smiled and spoke to them in a voice full of solemn authority.SB 10.89.57
Purport - Lord Krishna offered obeisances to His own Deity during the worship of Govardhana Hill, so now also He paid homage to His Vishnu expansion for the purpose of playing out His pastimes. The Lord is ananta, possessed of countless manifestations, and this eight-armed form is among them. He is acyuta, "never falling from His position," in the sense that He never stops engaging in His humanlike pastimes as a cowherd boy of Vrindavana. Thus to safeguard the special sanctity of His humanlike pastimes as Krishna, He offered obeisances to His own plenary expansion.
In this verse it is said that Shree Krishna bowed down before Maha-Vishnu ,which is he himself as leela of Bhagvanta , and that gives us the clear idea about relationships between both i.e. they are one and the same.
We can read the full story hereSB 10.89
There is no basic difference between Krishna and Vishnu. It all depends on Bhakti, and the preference of the individual, because as Rig Veda states
Ekam sat vipra bahudha vadanti
Meaning - Truth is One; the wise call It by different names.
Advaita Vedanta is solely based on the aphorisms of non duality. The Mahavakyas are the greatest proof of this.
Prajñānam brahma - Prajñāna (consciousness, wisdom, intelligence/Self) is Brahman. (Aitareya Upanishad 3.3 of the Rig Veda)
Ayam ātmā brahma - This Self (Atman) is Brahman (Mandukya Upanishad 1.2 of the Atharva Veda)
Tat tvam asi - Thou art That (Chandogya Upanishad 6.8.7 of the Sama Veda)
Aham brahmāsmi - I am Brahman (Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)
While Vedanta seeks to resolve the metaphysical mystery of this universe, it posits the existence of 2 forms of Brahman - Saguna and Nirguna. Advaita Vedanta considers "Atman" or the Self, as self-existent awareness, limitless and non-dual, identical to the universal eternal Brahman, whether Saguna or Nirguna. Saguna Brahman is same as Nirguna or "The Absolute" and is close to the concept of immanence, the manifested divine presence.
According to Advaita as taught by Shankara, Saguna refers to the Lord identical with his own infinite awareness. Shankara refers to him by names such as Shiva, Vishnu etc. This Saguna Brahman is eternal, undecaying and non-different from Nirguna Brahman.
In Vaishnavism, Saguna Brahman is given the higher preference because of the devotee's personal attraction to the manifested beauty of the Lord. The Universal Supersoul is formless yet all the forms are contained in it, as a result of the presiding Material Energy - Mahamaya, which in turn is controlled by the potency of Nirguna Brahman (Known to Vaishnavas as Krishna, Rama etc.)
Chaitanya Charitamrita, Madhya Leela, Chapter 20 Chaitanya Mahaprabhu says that Lord Maha Vishnu is an incarnation of Lord Krishna.
Chaitanya Charitamrita, Madhya Leela 20.250
Prathemei kare Krishna Purushavatara
Seita Purusha haye trividha prakara
In the beginning Krishna expands Himself into the Purushavataras, which are of three types
Goswami Tulsidas - Ramcharitmanas - Opening Sloka - Balkand
I adore Lord Hari, known name of Rama, who is superior to all and lies beyond all causes, whose Maya holds sway over the entire universe including Gods like Brahma, and demons; whose presence lends positive reality to the world of appearances even as the false notion of a serpent is entertained with reference to a rope and whose feet are the only bark for those who are eager to cross the ocean of mundane existence.
Bottom Line: It all depends on the devotee/sadhak/upashak/yogi etc.