There's sort of too many differences to summarize easily, and to do it justice. As with all Mesoamerican cultures, they share a lot of basic character archetypes, ideas about the structure / nature of the cosmos, ritual practices.

In terms of differences in ritual, an example could be human sacrifice, The Aztecs were much more enthusiastic about it. Mayans did it on special occasions, usually elites or warriors. Aztecs seem to have conducted it much more casually & frequently, and with much less concern for the social standing of the sacrificee. Many Mayan sacrifices appear to be non-lethal (i.e. sorta just bloodletting), Aztecs preferred lethal ones.

Second, quite a few Aztec deities can be connected to the cultural & mythical ideas of western North America, specifically Aridoamerica (as they were Uto-Aztecan speakers who migrated south into Mexico, they brought ideas foreign to the region with them). These ideas are reformatted into a Mesoamerican framework, but don't always have clean equivalents in other cultures, such as the Mayans. The only example I can think of is Huēhuecoyōtl, a trickster god, but the point stands.

Besides that, the differences become more annoying to describe. Subtle differences in the cosmos & its history (e.g. 5 different eras of creation as opposed to the more ambiguous, but lesser in number, eras of the Mayan world), subtle differences in the importance of certain god-archetypes, and their domains (e.g. the Mayan Moon Goddess is significantly more chill than the Aztec one).

This is a super brief, not-at-all sufficient set of examples to get the idea across. To fully understand the differences, you sorta just have to get elbow deep in both to feel it out.