There's some difference, but a lot of it is personality. The following descriptions are *really* general - I've heard of and personally had a lot of cases where one gender would (and has been) mistaken for the other on the basis of their personality.
Most females tend to be active, very nest-building-oriented, and more independent. Males tend to be more laid back, and can definitely be cuddlebugs.
But then, I've had a hyperactive female nester who was also an incessant shirt-diver, who was happiest when she spent four straight hours sleeping inside my shirt with me, and I've had a male who (but for his anatomical parts) was mistaken by a vet for being a female based on his behavior and body type!
It will depend a lot on the individual rats. But having companions, especially of the opposite gender (it seems to me) really energizes an oldie. Mr. Honeycomb (neutered male) was about 20-ish months old with mild HED, very depressed over losing his cagemate, when I adopted the Pips (three girls who were a bare two months old). He was so delighted by a) having company and b) having gorgeous girls to bug him and cuddle with all the time, that he acted like a 6 month old. And even though physically he began to age again (after what I swear was an improvement!), his mood remained elevated right up to his death.
