While it is possible to say that the Inca were in decline when the conquistadores arrived, it is much harder to say that about the Aztecs. Even with the Inca, it is not at all clear that they were in serious decline. Both empires had weaknesses, but it is not as if either was on the verge of collapse when the Spaniards came.
Neither empire had been in existence for a very long time when the Spaniards came. The Aztecs had been powerful for about 200 years while the Inca had only had an empire less than 100 years. Thus, it is not as if the empires were old and necessarily decrepit.
The Aztec Empire could be said to have been at its peak when the Spaniards came. It had no serious rivals in its area. There was not any important unrest. The empire was rather prosperous. There were groups within the empire who were unhappy at their subordinate status and who would help the Spaniards, but it was certainly not an empire in decline.
The Inca Empire was, at least at the time, in a somewhat worse situation. It is not clear that it was in permanent decline, though, as empires and countries in general often go through troubled periods. There was more unrest in the Inca Empire than in the Aztec. There had been uprisings and there had been fights over succession (though some of these were caused by smallpox spreading from the conquistadores).
So, the Aztec Empire was in better shape than the Inca Empire when the Spaniards came, but neither was clearly in permanent decline.