Comparison of upstream oil and gas automation solutions

I would really appreciate if you experts out there could give me your views on the strengths and weaknesses of the automation solutions for upstream oil and gas by the following companies:

- ABB
- Emerson Process Management
- Siemens
- Honeywell
- Yokogawa
- Invensys

I have heard that there is not much distinguishing the systems. At the same time, when looking at their websites, it seems that those applications that might be termed advanced solutions (asset integrity/optimisation, information management, process simulation, flow simulation etc) differ quite a lot.

Thanks in advance.