1)    Responding “Amen Yehei Shemeih Rabbah” is more important than responding to Kedushah, and it follows that if one comes to the synagogue close to Kedushah and if he would start immediately he would miss Kedushah but manage to respond to the Kaddish after the praying, and if he waits to hear the Kedushah he will be unable to respond to the Kaddish (and he is unable to wait until after Kaddish to pray as he will miss the last time for praying) it is better that he should start immediately so that he can respond to Kaddish. (Shulchan Aruch and Mishne Berurah 111)

2)    Responding “Amen Yehei Shemeih Rabbah” is more important than praying with a quorum, and it follows that if he is unable to interrupt for both it is better that he should respond to Kaddish and pray without a quorum. (ibid)