The comic-con trailer was so much better, showing Thanos as a threat that's been worth the long build-up and not just the latest Big Bad. I also liked how the comic-con trailer starts with Guardians (literally) running into Thor and pulls in the rest of the MCU from there.
I'm also not a fan of Captain America being in transition from Steve to Bucky. This is the final showdown. I want my Cap leading the charge, not playing the reluctant soldier.
This trailer by comparison looks safe.