To be as specific as I'm feeling right now, feel free to tell me to dial in further, and coming from having only played Arc Raiders and only watched Marathon vods here is the main difference - Marathon's devs are making a lot of promises vs Arc Raiders delivering on those same promised plans.
- For instance marathon is promising to launch with 3 maps, arc has 3 maps today.
- Marathon is promising tense extractor gunplay with high stakes loot, everything I heard from multiple streamers/reviewers say the tension isn't there and the loot isn't there. Arc has tension and definitely has loot. Their guns have clean 1-4 ranking and the weapons are rarity binned. I've yet to use most of the weapons available in Arc because I haven't focused on crafting them and haven't found them, theres already a ton of width to the pool.
- Marathon is promising strong PvE encounters with raid boss like content (or maybe the raid boss promise was actually people just speculating on where they could take this). Arc has boss like encounters with the Queen (and honestly fuck the Rocketeer and the Bastion those guys are tough little bastards that will punish you if you make a mistake).
- Marathon is promising dynamic events during the match. Arc Raiders already has dynamic events on a per map basis, night raids, and in server events like rocket landings, middle barages, etc.
I would pay $60 bucks today for Arc Raiders as it is now. My friends and I would play the fuck out of it. And if they would do DLC instead of battle passes we'd continue to financially support the game.
Based on what I heard and saw of marathons identical closed alpha, I don't know if there's enough content there for more than 10 hours and none of it excited me because it seemed like 20% of what they promised.
I think people are hyped by the concept of Marathon and the hope for an old Bungie game. But I think right now the reality is they're not the same Bungie as the one that gave us Halo, I personally never got into Destiny, and they've only gotten more corporate not less.
If in 6 months they can spin up what seems to be 80% of a game, then I'll be there and interested. But if Arc released next week, or spent the next 6 months adding content and I had to pick one, I'd be playing Arc without question.