Radar and infrared guidance have their own perks depending on the aspect.
Against an incoming target being attacked head on, you would want radar guidance, because that is where it works best. It takes advantage of the incoming doppler shift. On the other hand, if the target is receding, so will the doppler shift, and radar guidance is disadvantaged against rear end shots.
Infrared has the opposite. Since the frontal part of the aircraft don't reflect too much heat, attacks against the frontal aspect is seriously limited by range. On the other hand, the radiant energy from the exhaust are magnitudes greater than you get from radar reflection, so infrared guidance is the most effective with the rear shot.
Against an incoming target being attacked head on, you would want radar guidance, because that is where it works best. It takes advantage of the incoming doppler shift. On the other hand, if the target is receding, so will the doppler shift, and radar guidance is disadvantaged against rear end shots.
Infrared has the opposite. Since the frontal part of the aircraft don't reflect too much heat, attacks against the frontal aspect is seriously limited by range. On the other hand, the radiant energy from the exhaust are magnitudes greater than you get from radar reflection, so infrared guidance is the most effective with the rear shot.