Kumo no Mukou, Yakusoku no Basho, Steins;Gate Movie: Fuka Ryouiki no Déjà vu Recommendations
Both have same background story. However steins:gate giving more detail about their story on their series so its more make sense than Kumo no Mukou.
If we talk about romance , of course Steins;gate is no match for makoto shinkai. But if we talk which one is more entertain, Steins;gate is the winner
I suggest Steins;gate deja vu movie because the story was close enough with Kumo no Mukou but of course you need to watch the series for more detail.