Real trees are significantly better for the environment.
Benefits of real trees:
- An acre of fir trees can consume 12,000lbs of CO2. 
- For every Christmas tree cut down, tree farms plant one to three new trees to protect and maintain a healthy supply. 
- Trees are usually planted in otherwise unusable areas such as barren slopes, making them a good use of land.
- 85% of fake trees are made in China. Trees you buy at a local tree lot usually come from tree farms in your state. Not only does this help local US businesses, but this cuts down on the carbon footprint. 
Fake trees have one benefit: reusability. If you reuse a tree for fifteen to twenty years, you'll start to break even on the environmental impact.