Whoever makes the most money is one way to decide if job opportunities are about equal, but sometimes one partner's career choice is easier to accomidate than the other one.
For example, when my husband and I were first married, we lived in a city where there was a big pharmaceutical company. My husband was offered a job with them, but I was homesick and wanted to move back closer to home. I worked in a hospital laboratory and could get a job like that anywhere, but a job like the one I made my husband give up wasn't available back home. So, looking back, I probably should have sucked it up and stayed where we were.