I am not sure if I can tell you which is easier..... since I am allergic to cats. I have cat-sat in the past, but that is the extent of my cat-taking-care-of days. (It did involve feeding, changing the litter box, etc. Oh.... and I also worked part time at a vet clinic for two summers when younger also.)
I can tell you that fish do take work also, but there are also things you can do to minimize the work. Fish are more likely to die than a cat, so take that in consideration since you have a child. You may be having goldfish funerals in the toilet, or trying to substitute new ones in the tank while your child is in school, etc. Obviously, there is no need to walk or pet a fish. You will need to feed it daily, and change the water in the tank/ clean the tank. You also may need to monitor levels (such as ammonia, because you may need to add chemicals), make sure they are getting oxygen in their water, clean the filter, maintain water temp., etc. How much work you need to do is also dependent on how you set up your tank. You can purchase automatic feeders that will deposit a preset amount of food in the tank each day-- which saves you from having to feed them daily. You can purchase really good filters, which will not only filter impurities, but will also put oxygen back into the water, but then you also need to clean the filter (and possibly components of it, such as replacing carbon inserts if it is a carbon filter). If you do not have a filter then you will have to change the water and clean the tank more frequently, so it is easier to just have a filter. Also, if you do not have a filter, or depending on what kind you have, you will need an air blower to put oxygen back into the water. Cleaning the tank can take more or less time depending on if you have gravel in it, or decorative accessories, etc. Temperature is something you may or may not need to monitor. Some fish are more sensitive to temp. changes than others. It also depends on where the tank is located, and how much temp. changes in the location. You can buy a really cheap stick on thermometer that adheres to the side of the tank and will let you know the temp. of the water in the tank. You may not need to ever do anything to maintain it.
Some of the most resilient fish I have found are guppies. They are more resilient with temp. changes in the water. They also are eternally pregnant and eat their own, so if you have enough of them and forget to feed them, they will be fine. ;)
I may have forgotten some things above. Maybe someone who actually has a fish tank can tell you more. a lot of people these days seem to like those Beta fish, and from my understanding they are very low maintenance. I have never had one and honestly they seem quite boring to me and more of a decoration than anything. (They just sit there!)
