I prefer original characters simply because I'm a big fan of the Trek universe and I always felt that there were many other stories to tell about other characters than those which we have encountered on the screen.
There has already been plenty written about canon characters, crews and ships in Trek Lit and elsewhere and it's just fun to see writers adding and expanding to this rich universe.
In my experience the best stories achieve a good balance between engaging characters and an intriguing plot. Not that such a balance is always necessary. There are some terrific character driven stories out there as well as great examples of purely plot driven narratives.