Rigid borescopes give higher quality images, are easier to use and are less expensive than flexible scopes of similar quality. Choose a rigid borescope unless the rigidity is a problem, and always choose the largest diameter scope possible. A flexible fiberscope lets you see inside spaces that a rigid borescope can't penetrate. Viewing inside dark cavities is easily accomplished through a variety of optional light sources. For field inspections, high-power handheld rechargeable light sources work best. However, an external AC powered light source and light guide will provide the most amount of light.
You can find a good selection of rigid borescopes and fiberscopes at the Fiberscope.net specialized online store.