There's much more details in the animation... From the Navi American Troops actually bombing Japan, to the brother bringing his little sister to the doctor... In the original, the little girl gets sick, and you can feel the struggle. Here, she dies from not wanting to eat, which just isn't as impactful.
I can say that the acting is okay, and the story is fine i liked the film it was alright, but it's no where near as good as the original Grave of the Fireflies, that truly made me cry for 10+ Minutes. This didn't because we just didn't have as much development, and it's too similar to the original animated one.
I can recommend you sure go watch this one, but first watch the original, the original is a masterpiece that will break anyone's heart. I'm not saying that this movie is bad, it's just not as impactful as the original. The original hits you like a truck.
I am not saying that this movie won't make you sad or cry, but if you already seen the original, you kinda how what's coming and you are sort of ready for it to hit smack you in the chest.