Diva, ACE is one of the top five or six certifications. Almost any gym would accept their certification.
What people need to watch out for is the literally hundreds of bogus certifications available. You can pay a lot of money for a certification that's worthless.
Like Mel suggested, it's a really good idea to get in touch with places where you'd like to work when you're deciding which certification to get. Talk to head trainers about which certifications are held in the highest regard. I know that Ballys only accepts seven or so certifications, so if yours isn't on the list, you're out of luck.
Off the top of my head, I'd say widely accepted choices are ACSM, NASM (mine
), NSCA, ACE, and ISSA. But I'm not familiar with the group exercise ones.
Best of luck to you!