Apart from noise immunity, differential ring oscillator also allows you to use even number of stages in the loop of the oscillator. With single ended inverters, the stages have to be odd number if you want to get oscillations. With differential structure, you can just get the extra inversion by crossing the outputs of one of the stages. That also means that if you use, for example 4 stages in the loop, you can get 0, 90, 180, 270 degrees phases of your oscillation frequency.