I rode ride, mountain bike, and run.
Of the 3, I prefer mountain biking... but only with buds. I'm not big on mountain biking solo. It's more a social activity to me.
Road riding & running are SOLO Zen-like activities to me (altho I do go on group rides, really cool.)
I absolutely positively under no circumstances will use a trainer or a treadmill. It's just not for me. Personally, I find something very "fake" about it. Yes, you're getting a workout, while I'm sitting on my couch since I won't ride when it's 7 degrees outside!- but I hate hate hate it. Music doesn't help, and I would never watch TV while exercising- I'm trying to get in touch with my mind & body, not a reality show! Besides being boring, I feel no connection to nature, or my body for that matter. Hard for me to explain, I just can't stand the machines.
So, I run all winter (outside), and dream about riding when the temps start to rise. Every spring, riding is a fresh new thing for me.
If I lived somewhere with nicer weather year-round, I would most definitely ride year-round.