You know, I'd say back in the old days, adventure games were the shit, graphically. While RPGs used primitive (although charming) tile based graphics. Kings Quest V, which I believe was the first 256 color VGA game, or Mixed Up Mother Goose, when did they come out? Like 1989 or so? Looked much better than any RPG at the time, even Phantasy Star. Nowadays, the adventure genre is on life support and has been for a while, while RPGs are healthy as ever...
I remember absolutely loving some adventure games graphics back in the early days of 256 color VGA - Mixed Up Mother Goose, Kings Quest 5 and 6, Loom (Remember being really impressed with this), all looked really nice. While, Japanese RPGs still used tile based graphics and SD 8x8 sprites (Final Fantasy IV time period). Unfortunately, after Myst, the adventures seemed to go in the way of photorealism and eventual blandness. Although, I really liked Myst's graphics. Plus, some of the adventure games with the more animated/cartoony style (Kings Quest VII, a lot of the big Lucasarts games) looked great and still hold up if you don't mind the low resolution.
I think the first RPG that really impressed me graphically, aside from some cool monster designs in FF4 and 5, was FF6. The Phantom Forest just blew me away the first time I saw it, you know, the pond... Then, a lot of late SNES RPGs featured excellent graphics (Seiken Densetsu 3, Chrono Trigger, TOP, and Star Ocean)... but PSX is when the whole surreal worlds and beautiful artwork took off in RPGs - the FFs, SaGa Frontier 2, Legend of Mana.
I'm gonna concede a truce - both genres have had their share of memorable graphics and settings.