I see no reason to pick one or the other, it's not like anything stops you from watching both.
My shelves are full of DVDs of ST (movies and TOS), SW (all the movies), B5 (all of it), Firefly (all the little that was made), Terminator (all 3), Predator, Alien (all but AvP2), and so on.
None are perfect, all have flaws. What I find humorous is people putting down franchise A for flaws 1-10 while not acknowledging that franchise B's flaws 11-20 are just as great. Ratings and box office returns are a gauge of how popular it was, but not how good.
In the end, it's about what you enjoy watching, and that's like music or books or any type of art...it's personal preference.