Well, as you might have imagined, the question is pretty complicated, and it requires a lot of math, a little guessing, and patience. Lot's of patience. Because, depending on who you ask, you'll get different answers. That's the SF Chronicle's conclusion in their take on the debate.
Each option has it's own pros and cons.
• Artificial trees need no water.
• Some real trees are treated with pesticides.
• Fake trees are made mostly in Japan.
Read the full article for the whole discussion and a few ideas on minimizing the environmental impact of your tree (whether it's fake or real).
Image: Via Chronicle/Eric Luse