It depends on you goal for the project. If it's going on the web, you can capture your footage at the lower quality, since the full resolution will not be used anyway. You probably wouldn't tell the difference on an average 20" TV also.
Now, if you have a larger/higher quality TV, or you were planning to playback your video at full resolution on a computer, then you may want to use the higher setting, for obvious reasons.
I'd try both settings for practice. Like going to a park and shotting the same scenes at both speeds. Then compare the two on the computer and determine your needs from that comparison.
Have fun.