Gotham Knights gameplay looks far more in-depth and impactful, IMO. Something feels off about the Avengers gameplay I've seen. Though, I might be biased as a fan of the Arkham games.
I also forgot that GK is allowing you to approach enemies in various ways (like semi-stealth or going in guns blazing like Red Hood), which is a lot better gameplay wise vs Avengers, where it's you running in, and spamming a few QTE's.
I'm a fan of both Marvel and DC (but I think DC animations are superior to Marvel, while Marvel movies beat out DC), but I'm feeling DC games the past 5-8 years have fared better than Marvel's. I think GK is going to be the safer bet to buy into.