I've owned a gerbil, they are very cute, but the cage can smell, and personally i hate cleaning out cages. Just really depends on how much you want one.
I've never really had fury animals, because my siblings are allergic! so i've only owned fish. Fish are boring, as in the sense you can't pet them, but i've always loved aquatic animals. The pluses are is that its not hard to clean out tanks, keep them fed and happy if you pay attention to thier needs, PH, clean tank, food. Depending on the fish, it can be very cheep too, and pretty to look at. (sh.. if they die, you can always buy another one

) Anyways, u can always get different types of fish to match how difficult lvl, some fish like those fighting fish, u can just leave em there, feed like once a day or others, keep more of a cautious eye on them.
owning a small aquarium is fun, i've owned tiny little frogs at one point, turtles, newts, oh and even a puffer fish (so cute)