Kalki avatar would be one amongst us, a commoner who in times of need manifests supernatural powers whereas Lord Shiva never existed physically, is only an attribute of God Almighty, Brahman in Hinduism.
To re-establish Dharma (righteousness) on mother earth an avatar advents every 3500 years to 5000 years. In absence of an avatar, Dharma could never be re-established. Even a self realized soul like Mahavira, Gautama Buddha or Jesus Christ and Prophet Mohammed could do little.
Re-establishing Dharma requires the mantle donned by Lord Krishna, an avatar of Dwapar Yuga.
Lord Shiva allegorically signifies the state of self realization, god realization. Dismantling ego to zero, overpowering five senses and mind forever when human beings finally succeed becoming a pure soul atman, one finally becomes a Shiva, one whose kundalini has awakened fully.
The serpents in neck of Lord Shiva represent an awakened kundalini. Furthermore in the field of pure spirituality it is not the power exhibited by one that is of relevance but working for mankind at large all the time is the essential ingredient.
All man gods like Mahavira, Gautama Buddha even Jesus Christ and Prophet Mohammed reached the same goal, only their paths were different. None was more powerful than other. All worked for welfare of mankind all the time.