You should consider something that you are likely to use in the future. In North America, Allen-Bradley is the predominant brand, but there are others commonly in use. If you are in another part of the world, I think it's more likely you will be using Siemens but again, other brands are common. All PLCs function similarly as far as creating logic is concerned, but the development software is specific for each brand and many of them use different programming languages. This stuff isn't inexpensive, though. If you are learning on your own, working out of your own pocket, and just want to understand basic PLC functions, then I would suggest a low-priced unit such as what is available on Automation Direct or other online source. Even through that, you can gather a lot of knowledge and experience.
Brad Waybright
It's all okay as long as it's okay.