It's pretty tough to find an all-around harness that will fit every climbing niche. However, it is reasonable to find a harness for the type of climbing that you do the most and then use it as a multi-purpose harness. That said, different harnesses were designed for different uses; and if you elect to use a harness outside of it's primary application, it's likely that it won't work as well as you would like. Most mountain guides have two to four harnesses for different applications...
To begin with, you should be aware that there are three major harness categories. These are broken down into classes. A Type I harness is not a full strength harness and was designed specifically for rescue applications. These harnesses work like diapers and can be put on a patient quickly by rescue personnel. A Type II harness is what we think of for climbing. These tend to be recreational harnesses that are designed to wrap around your hips and legs. And finally, Type III harnesses were designed primarily for rope rescue and industrial rope work applications. These are full body harnesses with both waist, chest and occasionally dorsal tie-in points. It should be noted that there are a few recreational Type III harnesses on the market, most of which were designed for small children. (Small children don't have defined hips and often require this kind of harness so that they don't fall out.)
For the purpose of this blog, we are going to focus specifically on two broad spectrum types of harnesses. First, we'll discuss how to choose a rock climbing harness, and second, we'll discuss how to choose a mountaineering harness. If an all around climber has these two harnesses, it's likely that they will be comfortable in most settings.
Our friends at Origin Climbing and Fitness in Las Vegas have put together a nice video on how to choose an appropriate rock climbing harness
A mountaineering harness is a different beast. These harnesses are slimmed down compared to their rock climbing counterparts. They often don't have a lot of padding or extra gear loops. Indeed, they are designed to be very light with no frills.
Historically, many of the mountaineering specific harnesses didn't have belay loops. Such designs lead climbers to clip in directly to the interface between their waist and leg-loops. This causes carabiners to be cross-loaded, which isn't that big of a deal when the worst load an individual is likely to see is a crevasse fall. However, more and more people are using light-weight mountaineering harnesses for more technical climbing, including on steep pitched terrain. The result is that most modern alpine harnesses now have belay loops. It is no longer recommended that one purchase a harness without one.
Another thing to consider is whether the leg loops can be released or not within the harness. It's nice to have a set-up that doesn't require one to remove crampons or skis when putting on the harness. Fixed leg loop harnesses (which are generally designed for rock climbing) are very difficult to put on while wearing heavy footwear or crampons.
It's a good idea to try your harness on with the pack that you intend to wear. It's not uncommon to find that you get pinched somewhere you don't expect when you combine your pack with your harness. Most commonly this happens with gear loops in the back of the harness, but it could be from anything.
Finally, before purchasing any harness you should do your research. What do people like and dislike about a given harness? Does it work for the application that you intend to use it for? Does it tend to last for more than one season...? All of these are questions that you should ask, not just of people working at a shop, but of your friends or guides that have similar harnesses...
--Jason D. Martin