Should a light tunnel be positioned facing east or west?
There's no difference as your question is worded. The sun rises in the east and sets in the west, so half the day it will be on one side, and half the day it will be on the other.

Consider timing how long the sun is hitting each roof since there could be obstructions, a slight angle north or south, or other factor we can't help with here. Note that the sun will be shifting in the sky throughout the year, so don't be too precise with these measurements. Also consider what times of the day you'd like the most light, whether you're a morning person, or will be home and awake for longer in the evening.