Ah teenagers!! I also have a 14 year old, and sad to say but this kind of "defiant" behavior is what we are experiencing as well. Just an observation, but it seemed to me that the 16 year old was trying to stand up for you. He saw his brother not doing his chores and talking back to you, so he made a comment that started trouble. So my thoughts for you revolve around that observation.
First, talk to the 16 year old. Help him to understand that we are all different people in different stages of our lives. A 14 year old is (I think!) generally more defiant than someone who is older and may just be trying to assert his independence. Help your 16 year old to know that you will handle the parenting and that his comments are not necessary.
Then, talk to your 14 year old. Make him understand that his behavior will not be tolerated. Everyone has to contribute to chores and standing up against this is not "picking the right battle." Also maybe use the experience with the older brother to help him learn to deal with difficult situations and people. Talking things out or even stepping out of the situation for some breathing room are helpful techniques that he can use with difficult teachers, bosses, etc... in the future.
One other observation: If your kids are anything like mine, then the older one is convinced that the younger one gets away with everything (including getting out of chores) and the younger one feels like the older one is a bully that's out to get him. Make sure that this is not true. Also, you might try giving each kid a chance to air his grievances to you (rather than at each other). Just feeling like their complaints are heard may take some of the tension out. I know that they are brothers and that makes them prone to sibling rivalry, but as I tell my own kids: Siblings are the only friends you keep for your whole life, from cradle to grave. Best to try and make it work!
Good luck to you. I know that talking to teenagers can sometimes seem like you are talking to a brick wall, but it does soak in :)