My answer has changed over the years (and even sice my first post on this thread).
DP and I have talked about it, both in the short-term and in the long term and it's a really hard decision. Having one of us stay home ft would mean that we'd pretty much just "survive", but not get ahead financially or professionally, which is a concern for us. We want to buy a house and plan for retirement. I really want to be able to pay for the boys' college without them having to take out student loans and I'd love to be able to help them with a downpayment for their first house.
The older the boys get, the more I realize that mothering really doesn't end at age 5 or 15 or 20. There are things I want to be able to do for them throughout their lives. Some of those goals take money and some of them are more emotional & spiritual. I want to be able to provide material things for them, but I also want them to get drive and passion from me, as well. Working towards my own personal goals helps bring that passion and drive into our household. My 9 year old sees me studying or working late at home and we've talked about what I'm learning at school and what I'm doing at work and it adds to his life rather than take something away from it. I'm still there for him when he needs me and, except for rare occations like when I'm taking a timed test online or have a huge project at work, I'm always ready to drop what I'm doing and be there for him. I also still go on field trips with him and go to his basketball games, etc. I'm still an active part of his life, even when I work. I actually started really thinking about staying home a few months ago and was talking to DS about it and realized that his life wouldn't change at all, really (except that he wouldn't be able to do as many sports and lessons and things). He goes to my parents' after school and is waivered into a school that's better than our local school. He said he would want to keep going to his old school and go to g-ma's after school. SO, maybe I'd pick him up 30 minutes earlier, but that's it. He would have to give up things he loves if I stayed home. That was a wake up call.
As for DS2, I would love to stay home with him. Right now, the hardest part is figuring out what's best for everyone in the long run. Sometimes after a few too many hours at MDC, I start thinking that daycare must be scarring him for life. Then I'll have a day like yesterday when I drop him off at daycare and he doesn't really want to go, so I sat down and did an art project with him for about 20 minutes and he was happy and waiting for snack when I left him. He just kissed me and waived good-bye. When I picked him up, he was playing with one of the teachers and so excited because in his new class they GET TO USE THE REAL POTTY!!
I may not be there for every moment of his day, but I still get to share it with him, because he's my son and I'm his mom. He likes talking about what he did during the day and he's getting experiences that I couldn't give him. He has friends and tons of people who care about him. His world is bigger and different because I work, not better or worse.
In the end, I have 2 happy, attached, well-adusted kids and I'm a happy, well-adjusted mom.
So often I hear comments about staying home as if it's "only a couple years", but mothering goes on and on for the rest of your life. I want happy kids, but I also want to have a deep, lasting relationships with my kids as they become adults. As a grown woman the #1 thing that sticks out in my relationship with my own mom is that I feel that she gave up on certain dreams she had. I think she made decisions when we were children that she thought were right for us, but went against what she really wanted for herself. That, beyond anything, has made it harder for us to stay attached as adults. When my kids are grown, I want them to know that I loved them and gave them the best life I could, but I also want them to be proud of who I am as a woman. I want us to be close and I never want them to think that I am unhappy with my life because of things I did for them. So, I work hard every day to find the balance between my kids' needs and my own needs, so we all come out of this as guilt-free adults.