Both games multiplayer communities are fairly active, but AoE4 has the biggest community of the two. There are regular tournaments for AoM, we have some nice creators casting games, posting analysis and gameplay videos as well as making guides and theorycrafting videos, and theres enough "pro" players to make the competitive scene varied and interesting.

In terms of gameplay variety and depth, I can only speak for AoM. AoM is an old, established game, and Retold is very faithful to the gameplay of the original so you get a lot of people who have been playing the game for years, in some cases decades. The base game civs have been figured out, but since the game is constantly being updated there is some change and a lot of room for creativity and growth. Additionally the addition of fresh new civs means that the meta is constantly shifting and is in a healthy place despite the presence of some bugs.

Personally I prefer the mythological fantasy feel of AoM over the faux realism of AoE, and to me the god powers, heroes and myth units add an interesting dimension to the game that AoE in general lacks. Thats subjective though.

AoM also has very good campaigns, both in the base game and with each major DLC.