First of all, how do you measure love? I don't think it can be measured. It can change, grow, develop, and show in behaviors toward a person, but it can't be measured and compared. It shouldn't be compared.
A parent can love each of their biological children differently. That doesn't mean they love one more than the other, J. differently. Comparisons there can lead to hurt, resentment, and anger.
I have been a stepmom for 26 years. My two stepkids were 18 months and 4 years old when we moved in together, I had known them from babies. It was a difficult adjustment, and the 18 month-old made her dislike of M. very obvious! Her brother liked M. from the beginning. We gradually began to build a relationship. It took years. We are now very close and I am "Gramma S." to my grandchildren.
I also have a 9 year-old daughter. Do I love her more? Or do I love her differently? I'd say differently. The closeness that builds from pregnancy, birth, through caring for her as a baby, breastfeeding, etc. is something I did not get to experience with her brother and sister.
Also, a stepchild already has a mother. Even if I had wanted to be their mother, that wasn't going to happen. So, being a stepparent can be a loving, caring, parenting role. But it is not a parent.
A potential stepparent needs to respect your children and be willing to be a parenting partner with you. A close relationship may or may not develop over time between them and your child. I think you can look for a person who is willing to step into this challenging role, and to work through the adjustment that it takes, but you can't measure their love.