what is the difference between GDP PPP and the GDP nominal and which one is the best for the country .
PPP is for domestic purposes. Nominal is more for global purposes.
PPP is very important for people that live within a certain country because prices are not the same everywhere around the world. You get more for 100 dollars in China, India, compared to the U.S. or the UK.
PPP is also very important for a country that produces weapons and defence articles by themselves.
The country that does NOT produce much on their own, has to pay "the nominal price" for imports, which can be considered a "blood price" - meaning, it's extremely expencive.
For example, when Iraq imports weapons from another country, it has to pay "nominal price".
The same goes for the example of Russia, and the Mistral deal (if it ever comes through). The same goes for India. Whatever they import, is extremely expencive for India, since they have to pay "nominal price" - especially given that massive difference between nominal and PPP figures on India's behalf.
Those are just a few examples that are relevant for "Nominal vs PPP" question.