I went the other way, transitioning from composition to improvisation as my primary artistic medium. While the meaning of the two words may seem reasonably clear, I suggest that their manifestation in any individual creative psyche is extremely convoluted and deep. Neither is there any need for them to be mutually exclusive, no choice need exist at all, let alone be mandatory. Structure and form are arbitrary musical properties, just as this or that chord, this or that rhythm are creative options. If a priori, architectural structure is important to you, if you enjoy it, find it beautiful, then obviously you will lean toward composition and use improvisation mostly to generate ideas. If, on the other hand, you find yourself wanting to change things all the time, find it hard to settle on anything final, then you will lean toward spontaneity.
Until the last couple of decades and advances in recording technology, spontaneous creation could not produce an artistic end product of lasting quality. This obstacle no longer exists but, at least in my limited experience, very few musicians realise just how radically different things have become and how far creative choice has expanded.
The technical difficulty of notating improvised material in compositions has been discussed elsewhere and at length, by Andrew, me and several others. It is a separate issue, mostly concerns rhythm and need not be repeated. The only advice I have to offer is not to be hasty, do both, and keep all options open. The question will resolve for you in time. I was fifty-five before it did with me !