Can aggressive cats be thought to be friendlier and less aggressive?

I have two cats. One is as gentle and sweet as a baby and the other one is crabby and feisty. The crabby one gets jealous when I pet the gentler one but the reason why I don't pet the crabby one as much is because she has a tendency to scratch and nibble with her teeth, I'm just not sure how much petting she will allow before she loses it and I don't feel like spending money on hospital bills.