The rock climbing harness is the most essential piece of equipment a rock climber will buy, so it is important to choose one that is functional, comfortable, and reliable. The only way to test for comfort is to wear the rock climbing harness and hang from it, which is not always possible, so it may be difficult to make that determination. Look for adequate padding for the type of climbing you will be doing, and be sure the rock climbing harness you choose has an appropriate amount of gear loops.
Some rock climbing harness models are designed specifically for one type of climbing. Indoor rock climbing harnesses, for example, are usually lightweight, easy to take on and off quickly, and they may or may not be padded. This type of rock climbing harness is designed to be as unobtrusive as possible, allowing the climber as much flexibility as possible. The harness may not have any gear loops, or very few gear loops, as little equipment is necessary for this type of climbing. If you intend to climb primarily indoors, this is the harness for you.
Outdoor rock climbers will need to choose among several rock climbing harness models designed to do different things. A person who does traditional climbing, in which a significant amount of gear needs to be hauled, will need a well padded harness with several gear loops strong enough to haul the appropriate gear. These harnesses generally feature added lumbar support to accommodate the extra weight the climber carries, and it is very likely that this type of rock climbing harness will include an extra haul loop from which an extra rope can be secured. This is not a feature intended for anchoring use, but instead only for hauling use.
Mountaineers and multi-pitch climbers will benefit from a harness that allows quick adjustment to allow for clothing changes and for movement that will allow them to relieve themselves without having to remove the harness entirely. These types of climbers are likely to spend hours or even days in their harnesses, and removing them is not always safe or feasible. Specially designed harnesses allow the leg loops to be loosened and moved out of the way for such purposes safely and quickly. Such harnesses are often very adjustable to allow for different layers of clothing to be worn or removed.