Rats are very intelligent (many studies rank them at the level of dogs). They bond with people (some they like better than others), and they will run over to the side of the cage when they see you and try to get your attention. They can learn their names and various tricks. They also don't run away and will sit on the arm of a chair or a couch without running off (like a mouse would), though they probably will start exploring the area, they'll keep returning to you. They will also interact with you and try to get you to pat them and such, instead of simply walking on you like many other small animals. They aren't entirely nocturnal, and while they will be more active at night, they will also be active at other times of the day. They don't sleep in one large block like humans do.
A well taken care of rat lives about 3.5-4.5 years, 5 is pushing it and 3 isn't odd, though I haven't had one die that early barring a medical condition (I've had 10). Females are very prone to mammary tumors and spaying them can significantly reduce this.
They don't bite either, unlike hamsters. The only time they'd bite is if they're scared (been abused, rarely handled etc.) or they may nip if they think you're giving them food and it's only your finger (avoid this by making them see your hand first, instead of just immediately throwing in food). They also don't really smell, assuming they're cleaned properly (1.5-2 times a week).
Rats are extremely social and you really need 2 or more rats, not just one. Every single rat group (rmca.org is the largest) repeatedly state that and they really do much better in groups. You also should spend 30 minutes to an hour with them every day.
A 30 gallon tank is the bare minimum for 2 rats, and really is too small. They need a relatively large cage, preferably one with multiple levels.
As with all small animals, do not buy cedar shavings or any other aromatic wood shavings or scented litter. They wreck their delicate respiratory systems and greatly increase respiratory issues and also increase the rate of tumors. Buying cedar shaving will likely reduce the life expectancy of your pet, reduce the health of the pet, and increase your vet bills. Buy something like yesterdays news. It's a cat litter (but it looks like green pellets) and I pay 12 bucks for 30 pounds.
There's also a very cheap insurance policy available from vpi pet insurance for small animals. I pay $8 a month for my rats and it saves a lot of money. Basically, if an issue arises, you pay a $50 deductable and then 10% of the total bill, they pay the rest. Considering a tumor removal by a qualified vet can run 250-500, and the odds of any animal needing veterinary attention at at least one point is extremely high, it's well worth it.
With rats, or any small animal, make sure you go to a vet who is familiar with them. Taking a rat, ferret, rabbit etc. to a vet who is unfamiliar with them (many vets only really know dogs and cats) can be fatal for two reasons. One, they may not understand the symptoms of significant health issues and either tell you the wrong thing or not notice anything at all. Two, if they perform an operation (ie. tumor removal), they may botch it. One major issue with rats is vets not realizing that they need a heater to keep them warm while operating, since their bodies lose heat fast when under anesthesia. Not providing it can kill them.
Here's a more detailed faq on rats:
http://www.rmca.org/Resources/rmcafaq.htm
While most people think they smell (they are related to skunks after all), the biggest minus that you'd have with a ferret would be behaviour. They are incredibly curious animals, and WILL get into everything available. You can train them not to, but you're still looking at locks on cabinets, making sure all cords are tied up, and that piles of clothes aren't left out for long periods of time. Ex: Came home from work one night, and looked over at the couch only to see a ghost ferret. Went into the kitchen, and sure enough, he'd somehow managed to scale the counter and pop open the flour tin. He was trained, but since I'd worked extra long that day, his curiosity got the better of him.
I had one once and had to child proof my house. Child proof locks, plugs, everything. I also had to check the check when I sat down, out of fear of crushing it.
It also never learned not to bite (3 years old when I got it and I didn't really know how to train it). It would just run out from nowhere and try to tackle my feet, which would hurt.
\ ..esp fish)
ing day.
