The Steelseries uses a Pixart PMW3310 but the G502 uses the Pixart PMW3366 which is better. The Razer Mamba uses a "new" 16000 DPI sensor that I have no real info on. The only thing I know is that the G502 and Steelseries sensors are optical based while the Razer Mamba is laser based and everything I have read is that optical based sensors outperform laser based sensors.
I have a G502 Spectrum and it is a fantastic mouse but it depends on comfort for you. The G502 is more claw grip based while the other two look more palm based.
I stick with Logitech but the Razer Mamba looks decent.