Based on being a current user of both Discord and Slack for team collaboration, my preference goes out to Slack due to better support for threads to have more detailed discussions. There’s no ‘reply to’ functionality in Discord, which can make it very confusing when responding to something a few lines back. Furthermore Slack has nicer functionality for sharing blocks of code (e.g. highlighting).
A pro for Discord is it’s native dark interface, something in Slack can only be done through userscripts or something similar.