I think if someone checks the categories page in admin panel (if running phpld) you can sort the categories by number of hits. After checking few directories of mine, i found that Shopping is numero uno followed closely by Business category, followed by financial services. Travel is popular but not in top five. If it was one directory i checked, then we could have argued that more hits comes the way one promotes the directory and targets various SERPS. But since i checked five directories, the conclusion came much easier.
I find travel a very nice niche for a web directory, good interest and much potential within that niche, but I also admit, shopping isin't bad neither. Meti