I spent a long time weighing this one because I enjoyed both formats, and I think the choice depends heavily on what you want from the 'Hataraku Maou-sama!' experience. The manga adaptation by Satoshi Wagahara and Kurone Mishima is fantastic visually; it nails the comedy timing with exaggerated expressions and the action scenes are dynamic. You get the core story, the hilarious fish-out-of-water scenarios with Satan working at MgRonald's, and the charming character interactions. It's a complete and enjoyable package if you want a quicker, more visually-driven read.
However, the light novels offer so much more texture that the manga, by necessity, has to trim. Wagahara's prose allows for deeper internal monologues, especially from Maou and Emi. You understand their conflicting feelings—the residual hatred, the grudging respect, the confusing attraction—in a more nuanced way. The novels also include entire story arcs and character backstories that the manga either condenses or skips; for instance, the details surrounding the war in Ente Isla or the political machinations of the various angel and demon factions are fleshed out more thoroughly. The pacing of jokes can also differ, with the novels sometimes building up to a punchline over a longer conversational sequence.
If you've only seen the anime, the novels continue the story far beyond where the anime ends, exploring the consequences of Maou's life on Earth and the evolving relationships in a much more gradual and detailed manner. The manga, while it goes further than the anime, still hasn't caught up to the full scope of the later light novel volumes. So, if your primary drive is 'I need to know what happens next,' the original novels are your only source for the complete narrative. The extra world-building and character depth make the novels feel like a richer, more expansive version of the story I already loved. I found myself laughing just as hard reading the text descriptions of Maou's budgetary crises as I did seeing them depicted, which says a lot about the strength of the writing.