Real trees are the ecologically sound option. They are specially grown as a sustainable crop. When they are no longer required, they can be shredded and composted or used as a mulch.
Artificial trees cost energy to make, are non-biodegradeable and are often imported from far away (eg China), so there are high transportation costs.
Having said all that, if you already have an artificial tree, it is more ecologically sound to re-use it than to throw it out and get a real tree.
I have an artificial tree.