From a beginners perspective (i.e. my perspective) I find that both reading and writing are both supportive of each other and necessary.
They both stimulate your brain in different ways. Reading helps with learning and writing helps with memorization and construction.
On manga specifically, writing could be ok, but be aware reading could be difficult and/or misleading if the content isn't designed for learners. Learn Japanese from manga resources do exist, but I can't comment as I have no real experience with them.
[Edit: Sorry if I didn't really answer your question, its almost 4am here and I am procrastinating from sleeping]
You have to have lots of input from native materials. You learn to use the language like a native by observing how natives use the language. Practicing composition is important too, but you should have it checked by native speakers. But before you can compose, you need to be familiar with the language, so reading manga will get you there faster than just looking things up in a dictionary and putting them down. Dictionaries aren't terribly good with usage, and you most likely won't find the most natural ways to say things in a dictionary.