I prefer freestyle. You're free to express yourself or create something about anything especially anything that interests you. This is good because everyone has their own interest meaning the amount of work contributed here will be diverse and interesting.
When it becomes categorized or theme'd and everyone starts writing about the same thing it could get rather redundant / boring.
Of course these are just my opinions and obviously others would prefer a theme'd one over a freestyle and vice versa.
Perhaps you can alternate your Monthly Creative Writing Competitions? Example
July - Freestyle
August - Grepo Drama
September - Legendary Players
October - Freestyle
November - Greatest Battle Operations
And so on and so forth.
Because taking a poll will be taking the mass preference of the people participating. So if the outcome is always their preference when do they get challenged and when do they start exerting extra effort into making something?
Just my two cents