While there is no objective way to say who had a better life, it is very reasonable to say that poor whites in the South were better off than poor blacks even after the African Americans were no longer enslaved. Poor blacks and poor whites generally faced the same sorts of economic problems. They were both poor and they were often both stuck in poverty because of such things as the system of sharecropping. But even if poor whites were just as bad off in terms of economics, they were socially and politically much better off than African Americans were. Poor whites did not have to worry about race-based violence. They did not have to endure the pain of being on the lowest rung of the social ladder. They were able to vote and to be legally equal to other white people.
Neither poor blacks nor poor whites had very easy lives after the Civil War. However, poor blacks were generally worse off because they faced racism and its consequences where poor whites did not.