I've spent way too many hours scrolling through Tumblr and AO3 for these two, so maybe I can shed some light. In fan art, you get that single, breathtaking moment frozen in time—Aelin draped over Rowan's shoulder, both of them laughing; a close-up of their hands intertwined, covered in blood and dirt; Rowan's ice and Aelin's fire swirling together in this gorgeous, chaotic blend of colors. It's all about the visual impact, the mood lighting, the symbolism you can see. Fanfiction lets you live inside the space between those moments. You get the internal monologue, the unspoken tension, the slow build of a touch, the whispered conversations after a battle. Art gives you the 'what', but fic gives you the 'why' and the 'how'.
Honestly, I think the biggest difference is perspective. Art is often from an outsider's view, perfect and composed. Fic lets you sink into either of their heads, with all the messy, jealous, possessive, soft thoughts they'd never say out loud. I've seen art that makes them look like ethereal fae royalty, and then I've read fics that dwell on the sweat and the scars and the sheer exhaustion of their lives, which makes their quiet moments together feel even more earned.