I have kept rats for about 8 years now and have had males and females (dumbo variety) In my experience the females are a lot more energetic and unwilling to sit still, just like real life, and the males make much better lap rats that will sit on you while watching telly looking for an ear scratch. If you can put up with the slightly stronger smell and bulging nutsacks I'd go for a pair of males, I say a pair because you shouldn't keep rats alone.