Sounds familiar... Seriously, though, both ideas sound awesome. I'm personally leaning towards the first, though it might need something special to make it really stand out. With something like that, the key might lie in how society would build itself around the fantastical (magic, useful or dangerous flora / fauna, the interplay between the different races, etc.) and the mundane (widespread General Crisis-esque warfare, colonialism, advancements in art and science, etc.). Settings like Thief and Arcanum tend to do that very well, with at times literal conflict between magic / nature and technology / civilization. In Arcanum a wizard firing a gun will cause it to explode in his hands, while in Thief the two main religious factions embody each side both in terms of philosophy and aesthetic. Besides, I've been focusing a lot on the 18th century as inspiration for my current novel, and I'd love to see a setting reflecting the preceding century. That said, the other is still absolutely doable. Being set in the 20th century is the main thing that interests me, since in reality slavery was very much a no-no by then. That's not to say slavery isn't doable, but it needs to be handled right. I'd suggest being very careful about how society handled slavery, and think about it from the outset. My novel started out as Thief (Victorian era meets Medieval era in a fantasy world) meets Lucasfilm's Alien Chronicles (conflict between a downtrodden slave race and their decadent masters), but quickly found my own tone where slavery is more of an "out of sight, out of mind" affair and largely replaced by barely-better indentured servitude in developed areas. The brutality seen in Alien Chronicles didn't make sense in a Victorian era setting, and it shows in some of the early chapters. But at the same time, it all depends on what your interpretation of the setting is, so it may well be justified.