Its kind of easy to see as you progress,
a Obelisk is very cheap in terms of supplies (Zero cost) while Trees are very cheap in terms of coins - both only give 18 happiness. so depending on which commodity you are most well of in you take your choice.
but as time progresses Happiness becomes even more important and hence you pay a little more for more happiness, but then as you really understand the game you eliminate all that you can and build cultural buildings and only keep a few of the better decorations to fill in the odd gap here and there thats too small for a building.
But in the beginning a cheap tree or Obelisk is great. I still have about 4 trees from when I started tucked away in odd squares.