There are many differences between those two events. Firstly, Aang had to control one miniscule volcano, with the help of many people. On the other hand, in his past life, he had to control two mature volcanoes, which were probably twice as large as the ones Aang had to control. Also, Aang was not in the direct presence of toxic fumes, therefore he was not fatally affected. However, Avatar Roku received a strong gust of it which he had no choice but to inhale, therefore causing his poisoning.