Do both
I run, MTB, and road bike depending on what is coming up (and considering I'm planning on getting into multi-sport events soon, it'll only continue) or my mood. I think you'll find that a nice balance between running and biking is great.
From some reading that I have done lately, over half of the people who run during the year will stop because of injuries (this is in a book about running injury free and I've only started reading it, but...). Calorie-wise, running burns the most compared to cycling based on what I've seen and I do feel like I have to push myself more to feel beat after a bike ride than after a run.
And if you do like both running and biking, you can always take up duathlons (which is my goal right now). There are even off-road ones like:
http://bonkhardracing.com/races/berryman_du/default.asp
I agree with dminor though: for me, cycling often hits provides better anaerobic exercise where my running tends to work the aerobic range more. I think cycling has helped relieved some of my knee pain I get when running.