Which is good in hibernate HQL or SQL? When we can choose HQL? When we can choose SQL?

It doesn't matter too much - pick whatever you feel more comfortable with.

Personally, I find HQL to be overly fussy with case-sensitivity, but if you want your SQLs to be database independent, use HQL (or write ANSI-standard SQL).

I don't see any reason why you would want to make SQL standard practice.
If you use HQL, you can still fall back to SQL in those rare cases where you can't solve your problem with HQL.

Apart from that I simply prefer HQL over SQL cause of it's object-oriented syntax. And you don't need to specify how you want to join only THAT you want to join. (Hibernate knows how to join from the mapping.)

E.g.:
select c from Parent p join p.children c