I had pet rats for about 7 years and recommend them to anyone looking for a caged pet. They are cute, intelligent, affectionate, trainable, don’t tend to bite unless you’re talking about adult animals that have never been handled, and are much less stinky than most rodents. They also chew and can be destructive if not supervised, live a very short time (making it to 2 years is great), often die of conditions that are hard to watch (quick-growing cancer, incapacitating strokes); they are prone to serious respiratory issues, abscesses, and large benign tumors that require repeat surgeries and treatment. I had very good luck with mine, because of what I fed them I believe (real food only, not processed chows), but many fellow rat fanciers spend a lot of money on these little guys.
It’s great to get rats from a breeder. They will be properly socialized and come from stock which has been bred with the intention of minimizing health issues. They also have a huge advantage not coming from a pet store, that’s where most of the respiratory issues rats suffer with come from.
As for males vs females; males are larger, sometimes twice as big. Males are much more cuddly, females never stop moving. Males have coarse fur, produce musky grease on their back ends, and have a much stronger odor (I just bathed mine a lot and they smelled nice), and are kind of gross with their urine (pee where they sleep, dribble to scent-mark where ever they walk) while females are silky, have littler odor, and are generally tidy when it comes to pee although some dribble too - many can be effectively litter-trained in the cage. Both get along in small or large groups but it’s best to introduce them on neutral territory (and ideally as juveniles), otherwise there can be serious fights and injuries. I have never seen or heard of serious fighting and injuries in any but improperly-introduced rats. For most of the time I had rats I had a colony of males, I used to introduce new babies by putting them all in a clean bathtub together and then put them back in a freshly-cleaned age.
Rats should always be kept in pairs at least. They are as social as people and obviously much happier with a friend or two.
Another important bit of advice - it saves a ton of time if you keep them in a cage without tubes, or wire mesh shelves/stairs. Urine will coat everything and smell very strong. I kept my rats in big box-like cages with a ground floor only, covered with bedding, and they were odorless as long as I switched out the bedding/cloth hammocks on the regular (and cleaned off the hard objects in the cage, bowls/toys/igloos).
I don’t recommend aquariums for keeping, there isn’t adequate circulation and that’s hard on their respiratory system. Thirding that the type of bedding and regular changing is very important - aspen or one of the recycled paper options are really the only choices.