What you choose to learn is generally dictated by what is readily available to you. I started with the old Texas Instrument 500 platform because that was the only software in use at the facility where i worked. Seeing the same software used in different applications - conveyor control, batching, sorting etc - would continually reinforce my previous learning. Transitioning to other platforms -I’ve since worked with Siemens, Allen Bradley (500 and 5000), Omron, Mistsubishi - is easier once you are comfortable with one. Another factor is your location. AB is big in North America, Siemens is big in Europe (and with European integrators) .
The AB 500 platform is being phased out. That doesn’t mean you won’t encounter it in your travels. This software/hardware has proven reliable. Most people won’t upgrade until the scarcity of replacement components forces them to do so.