Aztecs were more complex with technology/engineering. While the Mongols were efficient also, The Aztecs still beat them considering they built more sophisticated projects. I mean these people manage to build an entire CITY... And create calendars as well.
Mongols were very focused on empire/society growth rather than food. They were very scarce on resources due to their choices on society. The Aztecs however used "floating gardens" to grow crops when land was scarce and that allowed the plants to flourish, which meant they were very resourceful.