Hello Arsal,
Fluency requires you to speak correctly. If you spoke wrongly ,than you are not considered to be fluent in the language . So there is no preference. Both fluency and correct grammar integrate;the more you use correct grammar in your speech , the more fluent you are likely to be . Of course fluency includes other factors such as rich vocabulary and correct pronunciation . So none is more important than the other.
Through your learning process on your way to get fluent you will be practicing to increase your vocabulary, to form grammatically correct sentences etc...