Most of the "big name" agencies have classes which are grossly similar, but can be run very differently. What you want to be careful about is not to choose the quickest, or the cheapest class -- going fast and inexpensive is not a good recipe for having reasonable class size so that you can get personal attention. It's also not a recipe for a class which is thorough, and gives you enough time to repeat things you are learning enough to make sure you have them down.
Also be careful comparing costs, because classes which look the same can be very different. One class may be $99, but not include materials, pool costs, boat costs, gear rental, etc. Another may be $425 and look horribly expensive, but include all those things.
You really do want a class that proceeds at a measured pace, with plenty of time in the pool (our class has nine hours, spread over six sessions) and unhurried open water days. The better the grounding you get, the more you will enjoy diving, and the more likely you are to stay with it, in my opinion.
