My wife and I are going with a 400sqft home kit from Shelter Kit
Its a traditional stick-build home (wooden 2x4s, etc.). They provide all the pre-cut lumber & fasteners & instructions, and ship it to your site.
You build it like IKEA furniture. Once its dried-in, you insulate the walls and finish it yourself.
For the size, its cheap! For eco-friendlness, pre-cut kits offer minimal waste, and tiny homes are eco-friendly anyway. We've had nothing but 100% positive experience with them.
We have longer-term plans to build a straw bale house, with clay-plaster floors and walls, but that's 5years out.