I think, Touhou has that Japanese old-world feel, with youkai and those fantasy elements that bring about people's imagination. Characters have their own backstory and/or legends, bringing about a great deal of speculation on their abilities, etc.
Seihou is more of a futuristic thing with those androids, machines and stuff. There isn't much history/stories to go from there, and I haven't seen anyone try to actually make a Seihou doujin (that would be really awesome for a change).
tl;dr - Fantasy > Sci-Fi in the terms of Touhou and Seihou.
Well the problem with Sci-fi futuristic stuff is that it lacks "history" to build upon of. Old fantasy monsters and creatures end up getting like hundreds of alternative stories go on while future stuff is all speculative.
Now while that does mean there is more "Freedom" to write what you want to write. It also means that there are alot of writers that actually have no idea where to go with it as there isn't a clear direction you can go.
