I realize this is an old post, but in case it is useful to others in the future, here is my answer. My husband is a surgical resident and we have one child, who was 1 at the beginning of residency. Our situation does not fall into the most common 3 categories listed above, and would be described as "working spouse, accommodating job." I am a lawyer, but I work a unionized fixed hours job with no travel. At my most recent location, I am extremely lucky because my boss happens to be married to a physician and understands residency. It is very hard to be the working spouse in this situation. My salary, which is greater than his, is essential for our ability to afford my husband's loan interest and my past loans in combination with our living expenses, but it is my job that has to give any time any issue comes up with our child. We do not have family available to help out without 24 hours notice. We make it work because I am willing to sacrifice advancement in my own career (now and in a way that affects any future advancement) because it is necessary to his career and we mutually came to this agreement. Like other people have indicated, you need a person who is able to be completely dedicated to your children time-wise and who can be flexible.
I take our daughter to a wonderful certified home daycare center during the days because she has health needs that make larger centers a challenge. We are fortunate enough to be doing residency in a state with relatively low daycare costs, and our particular provider charges extremely low rates. We could not afford a nanny here, even with both our salaries. We pay 40 percent of our combined income toward student loans to avoid ballooning interest. Our daycare payments mostly come out pre-tax through FSAs. To be honest, because she spends so much time at daycare during the week, all of my spare time is my daughter's. I do not take breaks for myself unless my husband happens to have the day off. I do not have a social life that is not related to parenting.
I could never survive having to live with another family member, it just would not work with my personality, background, and personal preferences. We tried family childcare initially in medical school and it severely damaged the relationships. The most important thing is that whoever the person who is going to be your support person is understands what they are getting into and is committed to making things work out, not an unwilling participant. Don't sugar coat or pass along sugar coated advice about how things will get better when X happens, because it's just a set up for disappointment.
When my husband is working nights, or night call, I take videos of my daughter and send them to him to watch when he has a chance. Sometimes he is able to FaceTime from the call room, but usually, on the rare occasions he even has a break, the connection is too poor or it is way too late. Some days he works longer hours than our daughter is awake. When I am with my daughter before and after daycare, I make a point to talk about daddy and what daddy is doing today and when we'll see him and how excited he will be to see her. When he is home, he is extremely involved with our daughter. He dramatically reads her favorite books, makes better funny faces than I do, gives her baths, etc. He works long hours, but at this point in time, I truly do not believe his relationship with her is hurt because when he is present, he is absolutely present.