The carabiners on the business end of your aiders will probably become your single most used piece of hardware. Choose them carefully. When you find a good one, buy two, because you need one for each aider. The aider carabiner should have the following qualities:
Non-locking - Sounds obvious, but I’ve seen beginners have a locking carabiner on their aiders. This is a carabiner you’re going to be opening and closing hundreds of times a day on a big wall, and you for sure don’t want a locking mechanism getting in the way.
Bent gate - A bent gate carabiner is slightly easier to open with a flick of the thumb. When you’re opening and closing that carabiner so many times, every little bit to make it easier helps. The bent gate also makes the carabiner opening a tad bigger, making clipping something larger like a wad of webbing easier.
Standard size - You're after the Goldilocks carabiner, not too big, not too small, just right. You don’t want a mini carabiner; harder to clip bigger random things. And, no need for a wide mouth belay style carabiner either.
Notchless - (aka “key lock”) to make it easier to clip and unclip. Fortunately, pretty much all modern carabiners are made like this.
Thin nose and thin metal stock - Having a narrow profile, or “nose” at the top of the open gate makes it easier to wiggle the carabiner into occasional small loops of webbing, rivet hangers, etc, and to unclip under load.
Distinct color - You can either buy an odd colored anodized carabiner, or even spray paint something creative yourself. Why is a distinct color important? You want to be able to glance your gear harness and always grab the aider fast. (Yes, you should get into the habit of always clipping your aider into the same spot on your harness every time, but having a distinctively colored carabiner makes it even easier to locate.)
Note that these carabiners are going to get pretty beat up, especially if you are clipping lots of bolt ladders, so check it regularly for any small burrs of metal that might hook up on webbing, and retire them if you need to.
The Petzl Spirit bent gate carabiner, a tried and true design, is a great choice.
thin aluminium and narrow nose
sexy French design! =^)