I'm not really a fan of the whole RPG system that both seem to be emulating, as well as the likeliness that Gotham Knights is going to feature MT's like Avengers is.
But seeing as how Gotham Knights isn't being designed as some weird MMO hybrid and offers co-op/SP, and to top it off, no console exclusive locked Heroes, I'd probably go for Gotham Knights (that and I like Victor as a villain).
Gotham Knights also appeals to me visually more than Avengers, but I'll have to wait and see what performance is like near it's release. My interest could be completely killed if WB do a shitty deal with EG though.
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"