Matt Ryan by far
Kaepernick has no pressure whatsoever. I believe it's all on the coach for making him the starter in the first place. Alex Smith was having a good year and almost took them to the Superbowl last year.
Why would Matt Ryan be under pressure (I know why, so as a fan it's easy for me to say what I say after this)? He knows what people expect of him. If he loses, it's not like he's letting anyone down. If he wins, a lot of people will take notice and start giving props. I'd say he's not under pressure, but just knows all he has to do is play football.