I have gerbils and rats. I like the rats much better, they're more handleable, a lot less bitey, a lot less psycho-dig-throw-fluff-everywhere. Way more personality... they're like little dogs almost. When you make a kissy noise they jump up on the cage expecting a treat... the gerbils are sort of oblivious to that sort of thing in my experience.

The gerbils are kinda cool to watch though. I'd take my aquarium of gerbils over one of fish any day.

To be honest though, I only got the gerbils because mice stink way too much to have around. They're more for the snakes than for me. I've grown to like them though.

Another thing with gerbils - the girls are super territorial, and they can go from best friends to sworn enemies in short order, usually resulting in one of them getting dusted... Rats don't do that either.

with either of them, you should have at least a pair... they seem sort of depressed when they are alone. no one to groom them and such.