Ultimately, you can't really compare them. One aims to advance the genre to score higher than high fantasy (whatever the fuck "avant-guarde fantasy" thing actually meant when they pitched it to Interplay) and the other tries to hijack the idea of a "detective RPG" to tell a story about some guy's torment by their haunting past. One tries to do more interesting things than average and the other doesn't really try to compete and just shows things they think are interesting.
Both of them feature really unwrangled writing, but with one you have someone who doesn't entertain the idea of an upper limit in writing (later on having his companion in New Vegas cut because his fat voiced ass was taking too much disk space) and the other being chronically online on Twitter and deciding that the entire game's dialogue style should read like a conversation from there. Former gets to absurd levels (especially with that UPDATED MY JOURNAL ding that caused me to burst into laughter multiple times during pretty serious dialogues due to how offhand it gets), latter can instantly ruin the moment when you recognize the particular Twitter character this caricature represents.
Both games have really good art with Disco Elysium being better on average but Planescape having these absurd spell animations that feel like interplanar bowling that win over me.
A lot of Planescape's combat could be streamlined even more. Disco's White/Red Checks, the Thought Cabinet and flavor descriptions for why exactly this one thing ups your skills were implemented a bit better, though the item descriptions in Planescape are also very nice.
Overall, I'd say that Planescape is the better game overall, but Disco is just bound to appeal to more people out there. It's just whether you get can get moved by more abstract ideas and philosophies in Planescape or more "real" and direct approaches in Disco. Also a shame we never got Lustmord's original OST for Planescape, Disco's OST is better.