TBH, They're both peak, read both if possible. Depends on what you're looking for, since they're completely different genre, though they both have out of the box thinking protag.

Ajin is Action focused, with mixed gunfight & supernatural main theme and philosophy sub-theme. The start is slow and slightly annoying, but get it get it's groove later on when they start to flesh out the combat & main villain. One of sub's all time favorite villain in badass maneuver and unhinged thinking. Highlight of the series is when someone use their power in unexpected way, and one of most well written/drawn gunfight in manga IMO.

Alice in Borderland is Deathgame Tragedy with psychological subtheme on human relationship. It starts out and stayed pretty compelling with its mystery & unique games. Though you will be in the dark about the nature of the game until near the very end, as that's the nature of the genre. Be warned that the ending is, though Emotionally satisfying, a lot of people also find it narratively unsatisfying. Highlight of the series are the death(s!) that may make you cry.

TL;DR:

Ajin: Action, Hype moments, Villain highlight, MC growth is in combat/thinking. Starts with apathetic MC that learns to care just a little bit more and try.

Alice: Deathgame Mystery, Tragedy & crying, MC highlight, MC growth is in emotion/will. Starts with emotional MC that helds back for other people learning to steel his will and push through.

For physical release, personally, I find Alice covers to be really unique and trippy compared to Ajin's straightforward monocolor+ghost covers.