I chose Geography over ES because ES is very heavily based on biology, and I had not taken this at A level. My opinion was that ES had too much science in for me and I wanted something more theoretical than practical.
I think Helloworld is right about the requirements being lower for ES, as the students doing ES at my uni are basically doing joint biology and geography rather than their own modules.
In terms of your career options, i don't think it matters too much between them. I preferred geography as i am interested in tectonics and geomorphology etc, compared to a friend doing ES who prefers topics on ecology and law.