Don't just think about how much sleeping space you'll get in a portaledge, but also how much room you'll have for storing your gear, if you need it. Portaledges come in single or double varieties. The doubles have enough room for two people to sleep -- or for one person and lots of gear. Double versions often come with "shark fin" dividers. Many come with loops sewn into the suspension straps, so you can easily hang your gear.
Another important element to consider is the rain fly. Some climbers like to risk a big wall excursion even if the sky is threatening a storm. In this case, it's a good idea to bring a portaledge with a reliable rain fly, even if you don't plan to sleep overnight. Others might want to risk an overnight trip without a rain fly, but experienced climbers say to always expect a storm and come prepared [source: Long].
Don't underestimate the importance of ease of assembly, either. If a storm is rolling in quickly, or you're tired after a hard day's climb, you won't want to waste time or energy putting together a complicated portaledge. Furthermore, if a portaledge has a lot of loose parts, they could be in danger of falling, making safe assembly impossible. Look for a portaledge with few loose parts and an intuitive assembly.
Perhaps the most critical aspect to consider is weight. It's easy to make your load too heavy by selecting a portaledge with lots of attractive frills and features. Instead, simplify your needs and look for a portaledge with a big space-to-weight ratio.
Some popular brands include Black Diamond, Metolius and Fish Products. When you're working with a portaledge for the first time, practice setting it up on a tree at a safe distance from the ground. (Speaking of trees, portaledges aren't exclusive to rock climbers: Big tree climbers use them as well.)
Above all, be safe. If you plan on cooking on your climb, don't cook in the portaledge itself. Instead, put any stove in a belay seat near the ledge, ideally clipped to an independent anchor.
With a reliable, lightweight, stormproof portaledge, you can really build a castle in the sky.