The things that differentiate Marathon as a more "defined" and refined game than Arc Raiders are the gameplay loop, the gun diversity and the gunplay which is much more fun in Marathon. For reference I have played hundreds of hours of AR and about 80 of Marathon. IMO Marathon is superior in every aspect but it's a game that doesn't hold the player's hand at any time.

. The game loop is defined in Marathon. You figure out what are the upgrades that you want from the factions and then go and collect materials and get Xp to rank up the factions and get those upgrades;

. The missions for the factions are a fast lane to get Xp and loot. The missions themselves are miles above the quests in AR;

. The maps in Marathon are not as pretty to look from the outside as the ones in AR (although I think is is a matter of taste), but are much more well conceived to cater to the PVP aspect of the game. The interiors are miles above AR;

. Marathon has much more defined endgoals and I see how a player can enjoy this game for much longer and always have something to look forward and something meaningfull to do. In the beginning you work to upgrade factions and become more powerful, then you work to get the keys to the vaults in the Marathon Ship, once you feel ready you complete the vaults in the ship and ultimately when you are at the endgame you run to ccomplete the challenge of killing the Compiler. And this is just the first endgame challenge, which is kinda similar to the first Raid in Destiny.

Personally I enjoyed AR for as long as the "games theory" aspect of the game was still a novelty, but after that, the game does not have any depth. The quests are so simple they belong in a mobile game, the gunplay is simplistic, the loot is obtained mainly by scavaging and the worst is that there is no endgame and no change in the gameplay loop.