If I have a tendency to start projects hastily, should I attempt to write a novel or should I make a comic/manga?

I take it you mean that you want to spend less time producing your work. If so, a novel will usually take less time. Comics typically take a long time to make, though it varies depending on what kind of art you are going for and what your skill level is. Some acclaimed graphic novels, such as Maus, were in the works for about a decade. There are some weekly manga artists such as Eiichiro Oda with a very high output but they have to work at a breakneck pace. How long it takes to write your novel can also vary, depending on your writing style.

But if I were you I would be more concerned with which medium fits you more than which will take less time. Which do you think you have greater skill in? Which do you think you would enjoy more? Which do you think can do a better job of conveying the specific story you want to tell? Those are more important questions.