I want to create a creative server but the only issue I have is figuring out if the map should be superflat or if it should be a natural terrain.
I highly recommend natural terrain, as it makes everything look much more interesting. You should probably use something like terrain-control or Worldpainter to create the world though, because that way you can optimize the terrain for your server.
In the end, that's why I moved to flatlands creative. Without heavy tools like VoxelSniper or WorldEdit, it can be hard for players to have an area large enough and/or flat enough for their builds.
I wouldn't say so. You could just make a high percentage of your world the plains biome, which is fairly flat, but you should also have some mountains etc., so people can build cool mines or other things that make use of the terrain.
I like it better. Here's a house designed to blend-in with the desert, and another house designed to be near the sea: