With a niche edit, the article already exists, you control only the link, and the cost reflects the traffic and authority of the host. In some cases an article that Google already knows can be re-crawled sooner than a new URL is found, but we give no timeframe, because it is not ours to promise. It suits supporting a page you already have.
With a new guest post, the article is written fresh, you control the whole piece, and the cost reflects writing and hosting. It suits covering a topic you have not covered. Our page on insertions or guest posts goes through when each one is the better choice.