One doesn't go slower than the other one. They still both tick seconds off exactly the same. The difference is that time is RELATIVE. It is a constant but only in it's frame of reference. So if you compared time compared to each stop watch, they would be exactly the same. However, when you drastically change the frames of reference (using high speeds etc) and compare the stopwatches against each other after one had its frame of reference changed, they would be different. Or to make it easier Watch to Time and they would both say the same thing. Watch to Watch it would be different, because they are in different frames of reference.