Pet rats aren't exactly high maintenance, but they do have their standards. Therefore, a number of things should be considered when making a decision on where and how to house them. Tails, for example. Rat cages that may seem more than adequate for the body length of your pet should also be spacious enough to allow room for the rodents' extraordinarily long tails. Otherwise, the animals may actually develop kinks in those tails.
Rats also get bored easily, so rat cages should include interesting, challenging and physical things for them to do -- ramps, slides and ladders. Companion rats are even better, since these are social creatures.
Generally, cages are better for rats than aquariums. True, the latter will make it harder for the occupants to escape (and rats are very good at that), but unless the enclosure is cleaned daily, the ammonia in the rats' urine will build up and could sicken them. Enclosed plastic spaces can also become dangerously overheated in summer, especially if placed in direct sunlight. Finally, the boxy configuration of an aquarium makes it difficult to include a rat gymnasium.
Rats enjoy being able to see and smell what's going on in the outside world, and that means slipping noses through the bars of rat cages. Make sure the cage is away from any drafts, and allow enough space for each occupant. Studies have shown that mice and rats require roughly the same amount of floor space, proportionally, as incarcerated humans. Of course, rats that are taken out regularly and allowed to exercise may not need as much exercise space in their habitat.
The rule of thumb, however, is "the bigger the better" when it comes to rat cages. Make sure the space between the bars is narrow enough to prevent escape (less than 1/4 inch for young rats, 1/2 inch for adults), and it's a good idea to find a cage with a substantial plastic base that will keep the rats' litter from being kicked out of the cage and onto your floor.
Adequate rat cages should also be human friendly. Make sure it allows convenient openings of sufficient space to import food and water and export litter.