Now that the manga has ended, I can make a review:
The original oneshot was a compact and simple story of two people falling in love, even though one is fated to die. The major difference in story between the oneshot and the remake is the ending.
In the original
the girl chooses to die to keep the one she loved alive, and in reality she had a terminal illness
In the remake
the boy ends up dying, she doesn't have a terminal ilness and his memory lives on in their child.
To be frank, the remake didn't add substantial development or plot that the oneshot didn't have. Sure it added more character development, but every page in the oneshot had served a purpose, and was paced perfectly, whereas the remake did falter in the development of the main characters, while sometimes getting distracted by the introduction of side characters.
Overall, a great read. A wonderful story of why some people choose to die for the ones they love, it helps you understand the motivation behind love, and the unreserved compassion that man can feel for those he loves.
That said, the story didn't gain much from being remade, the oneshot imho is the superior work. But well done nonetheless, Seo Kouji.