Rachel,
CSCS is better than ACSM in my opinion. I still like ACSM certs, but in my experience, hiring CSCS's I have found that they have higher practical application skills. NSCA (the National Strength and Conditioning Association) has the CSCS (Certified Strength and Conditioning Specialist) certification, which is no slouch of a test to pass either, let me tell you. Read most of your fitness magazines and look at people's credentials though, and you will see that more of them are CSCS's because that is one of the preferred certs out there. You have to have a degree just to sit for the test.