Krishna in Vrindavan is like God at home, while Krishna, as Vishnu in various Vaikuntha planets, is like God at office. A person at home is most comfortable and relaxed and has intimate loving dealings with his family members, while at office he has very formal dealings with his associates. Similarly in Vrindavan, all the Vrajavasis have very intimate dealings with Krishna in sakhya (friendship), vatsalya (parental) or madhurya (conjugal) rasas. No one there is aware of Krishna’s supreme position as God that usually results in very formal and reverential dealings. In the Vaikuntha planets, however, all the residents are always aware of Vishnu’s supreme position as the controller and maintainer of the whole creation. Therefore, all their dealings with Him—including those of sakhya, vatsalya and madhurya are tinged with the feelings of awe and reverence.
So, from an impartial study we can see that Krishna’s pastimes are more sweet and enchanting than those of Vishnu. In that sense, Krishna is superior to Vishnu.
Krishna also has four additional unique qualities not found in Vishnu. Srila Prabhupada explains them in The Nectar of Devotion:
1.Lila-madhuri—He is the performer of wonderful varieties of pastimes (especially His childhood pastimes). 2.Bhakta-madhuri—He is surrounded by devotees endowed with wonderful love of Godhead. 3.Venu-madhuri—He can attract all living entities all over the universes by playing on His flute. 4.Rupa-madhuri—He has a wonderful excellence of beauty which cannot be rivaled anywhere in the creation.”
Brahma-samhita compares Krishna to a candle from which several other candles are lit. These candles are compared to Lord Vishnu and the various incarnations. Diparcir eva hi dasantaram abhyupetya . . . . (Bs. 5.46) Although all candles have the same potency, the original candle still retains its uniqueness. Krishna is like that original candle, the source of all the incarnations.
After describing the various incarnations of the Lord, Suta Gosvami quotes the following verse (Bhag. 1.3.28):
ete camas kala pumsah krsnas tu bhagavan svayam indrari vyakulam lokam mrdayanti yuge yuge
“All the incarnations are either plenary portions or portions of the plenary portions of the Lord but Lord Sri Krishna is the original Personality of Godhead. All of them appear on planets whenever there is a disturbance created by the atheists. The Lord incarnates to protect the theists.”
Srimad-Bhagavatam also describes the pastime of Maha-vishnu stealing the sons of a brahmana in Dvaraka so that He could somehow get the darshan of Lord Krishna. This incident proves how Lord Krishna is the original Supreme Personality of Godhead and Vishnu an expansion. You may read this pastime in the Krishna book, chapter eighty-nine, The Superexcellent Power of Krishna.