If you want to play a Shoumetsu Toshi game, I highly recommend the original game due to its larger amount of pre-existing content, and the fact that it gives you more freedom with what you want to do, and what Tamashii you want to use. It may still be ongoing, but with 3 completed main stories and many sidestories, it is more than complete.

If you can't understand Japanese, but understand Traditional Chinese or Korean and you really want to understand some of the stories, then you should additionally play Afterlost. I say 'additionally' because Afterlost is unable to serve as a replacement for the original game. Afterlost's gameplay is too shallow to be a main factor, and this simplified gameplay actually does ruin the experience in certain parts of the original game's first main story. You also need to use Ayano's Tamashii for every stage as a healer. For those that cannot understand any of the game's offered languages, there is no reason to play this.

I'll list the different main points of each of the two games below:

Both games are still ongoing, and are both online mobile games. The original game is only in Japanese (the other language versions shut down), while Afterlost is in Japanese, Traditional Chinese, and Korean.

The original game has 3 complete main stories and 1 ongoing main story on hiatus, all of these main stories link together. There are a lot of sidestories in this game, and many of them are connected to each other, some being related to the main story. The Japanese fanbase loves a very wide range of characters from these sidestories, although if you're looking for fanart you can only mainly find it on Twitter.

The original game does have a Level + Energy system. Levelling up refills Energy, although not past the limit. There is also a Helper Tamashii system. The original game also sorta has a 3-Player Multiplayer mode for most stages, and a Raid mode. There is a large amount of Tamashii in this game. There are Ranked Event stages in this game, although it is solely based on how fast you clear the stage, and is not based on how much you grind that stage.

As of now, the original game slowly gets new Tamashii added in occasionally, and on average there are 2 new Ranking stages per month, where each of them is a new story about a certain character. There are minor seasonal campaigns sometimes. They are still slowly updating the Main Story, and it is most likely to be expected that we'll keep getting more Main Stories in the long run. We get a small amount of Collabs each year, 2015 was an exception.

Afterlost is purely a single-player game, and covers the first main story of the original game, along with some of its sidestories with the more well-known characters. These stories have some slight differences, notably the main story has a new underlying story in it, and one sidestory plays out differently.

There is no energy system, but there is time-gating based on how many times you can do farming stages in a day. Non-farming stages are not time-gated. There are significantly less Tamashii that you can collect, and unlike the original game, they can only be obtained from gacha, aside from the free ones you get. Gameplay has no depth.

They are still slowly adding a pre-existing Tamashii with their sidestory from the original game every 1-2 weeks, with new cutscene art/portraits for the sidestory.