I have had to show groom a lot of horses, many of whom were not mine and that I may not have seen until they walked into the stocks or cross ties. I always kept a lip chain in my front pocket and a twitch in my back pocket. Like Cherie, I prefer a wooden handle with a piece of rope in a circle, but the chain twitch or even the so called "humane or one person" twitch will work in a pinch. I'll even ear one if I have to, because I've got a job to do, very little time to do it (right before a show) and no desire to get hurt, maimed or killed.
At a show, I can't tranq the horse, it will fail a drug test. Frequently I have no idea how or if the horse has been well trained to accept the clippers anywhere, let alone near the ears. I'm real considerate about clipping ears, I put 'puffs' down inside the ear canal to keep the noise down and hair out, and I'll try to trim with my quietest, smallest pair of clippers. I'll give it 2 or 3 tries before I go to the lip chain, a couple more before I go to the twitch and a couple more before I'll have someone come ear the horse while I get the job done quickly.
I prefer the lip chain because if the horse acts up, it can be tightened and if the horse stops the bad behavior it can be loosened much quicker than a twitch. That way they get pressure/release based on their behavior and can learn from it.
I will always TRY to complete the job, but I also reserve the right to refuse to handle an ill trained or dangerous horse. I'm showing too and I'm wasting my money if I get hurt.