If you just are buying a carabiner for belaying at the gym or the crag, weight does not matter much. If you are climbing multi-pitch routes or doing alpine routes, every ounce is crucial. The difference between having six light locking carabiners like the Best Buy Mad Rock Super Tech Keylock Screw or Top Pick Wild Country Neon Keylock Screwgate Carabiner on your harness or six standard locking carabiners is big. More importantly, light biners are much less bulky like the small, compact Trango SuperFly Screwlock so they swing around less on your harness and don't get stuck as much in chimneys. The good news is there are more lightweight carabiners than ever. The challenge is finding a carabiner that can do everything you need at the right price. Most light biners work fine for general use, but do not excel at belaying, rappelling or use as a master point in an anchor due to their small size, except the Editor's Choice Petzl Attache 3D. You lose some functionality with most light carabiners so make sure the weight saving is worth it. We discovered that the manufacturer's quoted weight and the weight from our scale can differ by as much as five percent. This doesn't mean you should trust our weight more than the manufacturer's. But it does mean you should not get hung up on a few grams of difference when buying a light carabiner.
Gate Type: Screw-gate or Twist-lock?
Screw-gate carabiners are usually 25 percent less expensive and lighter in weight. In general, twist-lock carabiners are best for belaying and rappelling and screw-gate carabiners are better for everything else.
When buying a screw-gate carabiner, consider how many twists it takes to open and close the gate. We generally prefer biners that take fewer twists because you save time like the large DMM Boa or smaller Wild Country Synergy. However, some people may feel safer if a carabiner takes more twists to open and close. Also, when carabiners are brand new, some gates will be more fluid. Some biners can be opened and closed with one precise flick of the fingers, which is convenient.
Because you have to remember to close a screw-gate carabiner, it's nice to have safety features like Petzl uses on many of their biners like the Petzl Am'D. They have a red stripe that disappears when the gate is closed. This helps remind you to lock the gate and also helps prevent over-tightening. Other manufacturers deal with the over-tightening issue by putting metal on the gate so that the gate stops the screw-gate mechanism before it hits the other side of the biner.
We prefer key-lock gates because they get hung up less on slings and bolt hangers like we found with the Black Diamond Mini Pearabiner or Wild Country Neon Keylock Screwgate Carabiner. Most carabiners now come with key-lock options and they generally don't cost more so there is no reason not to go the key-lock route. In addition, some key-lock carabiners are a little smoother to get on and off due to the angle of their key-lock; the smoother the angle the smoother the carabiner comes on and off. However, that is a pretty subtle distinction between key-lock carabiners. The main thing is to go key-lock when possible.
Number of Knots Held
If you use a carabiner in an anchor, especially as the master point, it's nice if the biner holds lots of ropes and slings like the Black Diamond Rocklock or the DMM Boa. Generally, the larger the carabiner, the better it is at holding lots of cord. That said, shape also plays a role. If a carabiner has a pear shape with a big area below the gate, then more ropes and slings will fit in. A D shape or oval shape generally holds a little less. Lightweight carabiners hold very few ropes and slings because of their compact size, which limits their versatility.
Rope Pull Smoothness
How smoothly a rope pulls through a carabiner is essential if you are belaying a second climber off the anchor using an auto-block belay device like the Petzl Reverso 4. The wider and rounder the stock of the aluminum, the smoother the rope will flow over it. In addition, carabiners like Petzl OK or Petzl Attache with more oval shapes or pear shapes generally keep the rope from contacting the spine and therefor have less drag. The other instance where rope pull smoothness is nice is with top rope anchors, where it is nice to have a smooth pull. Other than those situations, smoothness is less important compared to other factors like the gate performance, shape and weight.
We found gate clearance to be less important than how many ropes a biner will hold. In general, you are only clipping one rope or sling at a time and all gates work about the same. Only in very specialized applications like using a locking carabiner with a big pulley is gate clearance as big an issue.