What an adorable house! And you've done beautiful work on it. I voted for "horizontally, across the top". For one thing, it's the easiest to see and to read. And aesthetically, I think it just makes the most sense. While I understand what yoboseiyo is saying, I don't think it's necessary to fill every space with something. However, if you want to, you could add another, larger plant/planter next to, but slightly behind the plant on the right (as you are looking at the picture). The house itself is not symmetrical, and I think you need a little something else there to help balance the front. Have fun as you finish your home!