I mostly agree with Graham, and the operative words of his post are "new idea".
The slightly tentative response is because I suspect it may be too soon for you to know for certain. Write what seems to be working best for you and the story/stories. Worry about what happens later, later.
Besides, short story collection and novel are
not mutually exclusive. Think of Asimov's Foundation: originally published as separate short stories, but were eventually assembled into very satisfying novels. And he also came up with new ideas for full novels later. You can have it all, if you keep an open mind.
