Verdict
Duolingo Chess is best for absolute beginners learning the rules; Mainline is best once you're playing real games and want to improve.
Duolingo Chess teaches how the pieces move and basic concepts in a gamified path. Mainline is the next step: it keeps the daily-streak, bite-sized feel, but trains a real improving player against real opening lines and your own game mistakes. If you've graduated from "how does the knight move" to "why do I keep losing in the opening," Mainline is built for that stage.