A planet just like Earth (with the same mass and same composition) but without photosynthetic life would have an atmosphere consisting of nitrogen, carbon dioxide and water vapour. The increased level of carbon dioxide, which is a greenhouse gas, would make the planet warmer; Lifeless Earth II would be entirely uninhabitable, although it might be possible to terraform it.
A planet just like Earth (with the same mass and same composition) but with alien photosynthetic life would have an entirely unknown ecology full of possible pathogens and allergenic compounds; the alien life-forms are very likely to be either poisonous or unpalatable. Life-bearing Earth II would be a potentially dangerous place to colonise, although it may be possible to adapt either the planet or the colonists to one another with some difficulty.