Having worked with many PLCs over many years, and even having to make my own, I can pass on my recommendations - in order -
1/ Check with the Industry you are working in. Often, they have a preferred/common brand/system. It saves a lot of hassle when you are with a brand that
your Industry is already familiar with. I often "borrow" expensive development tools, as some brands can be a huge rip-off.
2/ Determine the number of I/O you are expecting. Including opto in, relay out, analogue in/out, timers etc etc. Some brands charge a fortune for every type
of module, and you end up with several, just to do simple jobs.
3/ Price can vary a LOT. I often use OMRON, as they are quite cheap, have simple ladder programming (with FREE versions), and a good selection of modules.
Price is also important with the programming kits / software. Some rip you off good, for bloated over-done software / Interface cabling.
4/ Availability. If you have a very good local supplier / support company, it can make a huge difference.
Hope this helps