I think it would be more difficult to be a working mother, personally. Not only do you have to go to work all day, but then you'd have to come home and take care of everything around the house AND the child. As a stay at home mom, at least you have a longer amount of time to get everything done.
The issue with being a SAHM is that the majority of people think if you don't punch a clock (so to speak) you have it easy. In some ways SAHMs have it easier, and some ways it's much harder. Working moms get that break away from the kids and house that SAHMs don't. Grown up time is a valuable commodity for SAHMs, the same as quality child time is to working moms.
No matter what choice you make, you are going to gain some things and lose somethings..........you have to weigh out what is most important to you and your family.