Here is Wikipedia's definitions for Hyrulian.
http://en.wikipedia.org/wiki/Hyrulian
And Hylian.
http://en.wikipedia.org/wiki/Hylian
Scroll down for the definition of Hylian.
I think both terms are accepted for use, like other real races have different terms of use.
EDIT:
Hylian - The term "Hylian" generally refers to a member of the Hylian race, who are sometimes capable of using magical abilities.
Hyrulean (correction from Nintendo though the term Hyrulian is also applicable) - "Hyrulean", on the other hand, means from or related to the mythical land of Hyrule. Note that not all inhabitants of Hyrule are Hylian: only those with pointed-ears, such as Link, are Hylian. Unlike Hylians, Hyruleans are sometimes not granted with magical abilities, but they do not have pointed ears.