Back in the day, Game Freak used to make an enhanced “third version” for every generation of the main Pokemon RPG series.

For instance, Pokemon Yellow was released as the “third version” to Pokemon Red and Blue, and Pokemon Platinum was the “third version” to Pokemon Diamond and Pearl. This tradition was broken when Game Freak developed Pokemon Black 2 and White 2 as sequels to Pokemon Black and White.

In an interview with GameSpot, Pokemon Ultra Sun and Ultra Moon director Kazumasa Iwao explained why they decided to make Pokemon Ultra Sun and Ultra Moon as two new games instead of one.

This time around there are differences in the story between the two versions that lead us to a split. But one particularly appealing aspect for us in creating two different upper versions is that we can get many more different Pokemon and characters into the story and into the game itself. If you try and force all of that into a single upper version then the relationships between characters and their interactions they have can start to get a little bit confused and a bit disjointed, whereas if we separate them into two versions we can really focus in and make sure there’s a strong core to the story. Splitting into two versions allows us to draw the most from the characters in the game, and hopefully players will enjoy that.

As for whether you should get Ultra Sun or Ultra Moon, here’s what Iwao and Pokemon Ultra Sun and Ultra Moon producer Shigeru Ohmori have to say.

One point users might want to consider is that the Pokedex in Pokemon Ultra Sun is based on the Pokedex in Pokemon Sun and the same for Ultra Moon and Moon. So if something the players are really looking for is Pokedex completion, they might find it better to go for the opposite version this time round in order to help complete their Pokedex.

Equally if you’re looking for a story-based experience then those who played Pokemon Sun may want to go for Ultra Sun as there are some small details that remain consistent throughout the two that might appeal to people looking for a story-based game.

Pokemon Ultra Sun and Ultra Moon launches November 17, 2017 on Nintendo 3DS worldwide.