Each Vendors market varies with region availability. For example, a good share of Allen Bradley is in North America, especially in USA and Canada, while Siemens has a good share in Europe and Middle East.
Siemens is the most commonly used PLC followed closely by Allen Bradley. SIEMENS AG provides PLC to Chemical, Petro-chemical, Off-Shore, FPSO and BMS industry. It is much recognized because the PLC processes inputs with a fast response time. But the negative point on their front is that during the functioning if even one component fails to act properly then the system becomes unavailable as a result of which the machine stops functioning which may lead to further issues.
Rockwell Automation is another company with reach to the Chemical, Off-Shore and FPSO industries only. The strength of their provided PLC is that it has fully structured programming software along with the facility to manually select the outputs requiring shut down. However, its drawback is that the structured programming is not favorable to process safety critical circuits which can be a problem.
Delta is an upcoming brand and is popular due to its communication capabilities and price. It has also got the good performance where the temperature in the industry is high.