Method for make map

Hello! I want to make some side scroller shooter, and I want to to ask you what is better method to make map? Build big size level in unity editor or generate in run time map ? I want to make something like this: alt text

I'd use a 3d modeller to create tileable pieces that you can put together in unity, or make primitive planes and texture them, making sure they face toward the camera at all times, and make sure they have colliders.