Which Ex personnel competency system shall be chosen?

Many systems are available on the market. At the end of the day all shall have the same level of knowledge, understanding regarding explosion protection.

But again which system to choose?
You shall choose the one which is requested by your customer, by the country, by the region. Furthermore the one which covers more your needs. No other way around!

It may happen that 3-4 methodologies you may apply for being well accepted worldwide.
Citation from IEC 60079-14:2013 4.5:
“Competency can be demonstrated in accordance with a training and assessment framework relevant to national regulations or standards or user requirements.”