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.
Step right up come on in, feel the buzz in your veins, I'm like an chemical electrical right into your brain and I'm the one who killed the Radio, soon you'll all see
So pay up motherfuckers you belong to "V"