I recently did a video on this! If you like, my channel is "whimreaper" on YouTube.
Alpha CC is designed to look more realistic. It often consists of several categories of skin details and accessories to achieve a really put-together aesthetic. For example, alpha cc shows individual hair strands, has realistic eye overlays, and features categories that are not a part of the game algorithm (like painted nails and separate jackets).
Maxis Match CC is designed to fit into the game graphics to blend seamlessly with the surroundings. This means the content looks like it was made to belong in the Sims 4 (AKA Maxis made) and cannot always be distinguished from game content. This content tends to stick to the major clothes/skin detail/accessory groups that the base game offers and its main purpose is to provide some variety to players looking for more content whilst fitting in with the other objects in the game.
I hope this helped :) You can see me give examples in my video (just search my channel, and it should be on there)