There are many factors, but I bought the inexpensive Nashbar Waterproof ones and found them quite adequate. Between three of us, we used two sets of rear ones, two sets of front ones, one set of nashbar mtb panniers, and one set of Transit Epic from Performance. They all held up fine for the duration of the TA and some shorter trips. One set of the waterproof ones has been used for daily commuting and is holding up well to that. Performance also sells the same ones. I think you can usually find them on sale and often get 10-15% additional off. I think the 15% off everything is in effect this weekend.