You need to do what is right for your FAMILY. The moment you and your husband got serious about your relationship it stopped being about you and started being about WE.
You are acting like children are a burden, something awful that is preventing you from doing better things. There is no greater calling than parenthood. You are developing the next generation, the people who will be running this world after we are gone.
Your career is secondary to your family. On your deathbed you are not going to wish you had spent more time at work. You are going to wish you had spent more time with your family.
There's nothing wrong with taking less demanding jobs out there. Less demanding means less stress! You can still have a challenging job that does not infringe upon family time.
If you have a third child you will love that child, and that child will be a blessing to you. But your focus should always be family first.
Talk to your husband, and LISTEN to your husband. Listen to him. He has valid points as well and he deserves to be heard.
EDIT: Anyone talking about over population are just plain wrong. I have studied ecology and the problem is not that there are too many people, the problem is that there is an uneven distribution of resources. There are also MANY couples these days who choose to have no children (there are even special groups for it), and MANY couples who choose to only have one or two children so it balances it out here in the USA. There are actually countries, like France, who are desperate for immigrants because there are not enough of their own people having babies to support their economy or military because people are too busy to have kids.
About adopting an older child. Usually those kids have been severely abused which is why they are in the system. You will have to deal with all sorts of issues and may actually require MORE work in the long run. I've got nothing against adopting older children but you've gotta be prepared for the issues that come with it.
Remember, it's not about you anymore. It's about your family. Family First. (That's also a book, I recommend it.)
Oh, and I am the third out of eight children. My parents made sure that we all got individual time with them, especially my Mom. She was a pro at it. We also all get along with one another and enjoy family visits.
PS again, Abbie H had a fabulous suggestion. You and your husband should be a team, and that's how a team figures out their next move.