in my opinion, a cat is easier to take care of than fish.
cats do not need very much attention unless they are kittens. all you have to do is feed the cat, occasionally change the litter box, and give it love. what i do is i just keep a whole tub of their food out so they can eat whenever they need, and you can also buy those self cleaning litterboxes (very expensive though). cats arent usually messy as some say, or at least mine arent. sure theres the occasional hairball or something ripped up. but if fed well and treated with care, they will be on best behaviour.
Now with fish, you would think "they cant go anywhere, its too easy!" but if youre thinking on getting something other than a Beta fish, its hard work, and EXPENSIVE. i just have Loaches at the moment but used to have many others. a tank that needs a filter and a heater usually cost over 50$. and cleaning tanks that are 10 gallons or more is a very big process. changing water, cleaning the glass, changing the filters, adding water conditioner, etc... for me it takes around 2 hours. and i have to do it atleast one every 1 or 2 weeks, or algae build up becomes insane. BUT if you are just looking for simple fish, get a beta, they barely need ANYTHING. goldfish are somewhat easy as well, the downside is that they cant be with other species of fish (they will eat them,) but the plus side is that you can have more than one goldfish unlike betas.