The bottom one will easily be seen if the robot flips over as did in the UTC match 8 video

But the judges can't penalize you there. The numbers are visible from 100 feet away. Whether at all times is a different issue. Basically, for teams who can't get their numbers on all side, the top is a good side to use and the bottom is a last resort. I'm just trying to help those people who didn't read all the rules carefully.