It really depends on what your current backgroud is, and what you want to do with it. For background, I’ve had the ACE cert in the past and am currently CSCS certified.
If you simply want to open the door to getting a job at a gym as a personal trainer (ie: tennis moms who want to keep that nice tight butt, the new year’s resolution crowd that wants to lose 10 lbs, etc) then ACE is fine. It’s not the best, but it’s simply there to give you a background in designing simple exercise plans and a simple overview in some of the basics of nutrition. However, if you want to do any more than that, go the ACSM or CSCS route. Granted, these take a good bit more time, preparation, and background, they are much more highly regarded. CSCS is what most college strength and conditioning coaches are required to have. Addtionally, team trainers for MLB teams have to have a CSCS. My undergrad degree was in exercise science, and I saw the CSCS test as a cumulative exam of my 4 years of course work. It better prepares you to design more complicated training routines, and better understand the metabolic and nutritional demands of the exercises your clients are doing. The certification should indicate that you are much more capable of designing functional exercises (ie: helping that same tennis mom develop more power from her hips to create a more powerful serve so she can go from playing at a 3.5 to a 4.0 level), compared to ACE which should show that you know what exercise to prescribe to build bigger biceps.
I would expect to hear from others who have their CSCS in this thread that they view ACE and other similar certifications as trash.