danielstacey, welcome to the world of PLCs. Your question is very broad and needs to be sharpened a bit to your specific need and then someone here can help you.
A PLC is a PLC is a PLC. No matter if it says Omron, AB, Siemens, or a few dozen more brands on the front they are basically the same at the core of what they do. Some use Intel, Motorola, some use other RISC based processors, some use commercial OS and others use custom developed OS, but they all use some type of RTOS at the core.
A PLC is best used for discrete (on/off) control no matter who's PLC it is. Some PLCs do process application better, but for true process applications it is best handled by a DCS system.
PLCs come in basically three sizes, small, medium, and large. The best fit to which PLC is determined by the application you need one for. Stand alone, or networked also determines which PLCs you can use. Motion cards, basic modules, and other specialty cards also determine which PLC you can use.
As for who's PLC to use? Well that is a whole new subject, and being that I work for the world wide market leader in PLCs I can be a little biased to say which brand works best, but it really depends on your needs, likes, and local support. Do your research and study different resources available on the web and in hard copy to learn more about PLCs and their pros/cons. Good Luck
Michael