Its hard to say which one will burn more calories... I've known some people who biked seriously as a sport, and they said that training inside is never the same as training outside. The way the stationary bike can change difficulty levels is never the same as hills that nature provides, so people biking outside will have more stamina and preparedness for a race then people training inside. Having said that, once you're at certain level, using higher gear may not be always possible, because biking as fast as you can is sometimes difficult on a busy bike path, so you can't train to the maximum of your ability.
I think calorie burning number depends a lot on terrain and whether you're able to go at maximum of your ability on the street vs. at home. But all in all, I would not think that the difference in calories burned would be that significant if you train at about the same level indoor or outside. I think the final decision should be your preference. Personally, I enjoy biking outside way more because it provides balance challenges, going around obsticals, and variety in terrain and sights. Stationary is pretty boring, but when you can't go outside due to weather, its the next best thing.