My choice of pack is usually dictated by our need for water.
For urban caching, I have a small utilitarian bag. It has a shoulder strap, but is not stylish enough to call a purse. It has a few pockets to keep small items sorted, and has plenty of room for a small camera, GPSr, some swag and even a water bottle.
For day hikes, I use either a small hydration backpack or a regular school backpack with a hydration pocket built in, depending on my need for other gear. Both were purchased before we bagan geocaching.