Traits vs injecting a class through the constructor

I'm trying to work out when I should use a trait and when I should inject a class through the constructor.

Which is the preferred way to use service classes? The example I give is stripped down to get a feel for the problem without pasting too much code in the OP.

Solution A: Use the IoC container to inject an instance of the service layer class

class ActivationKeyGeneratorService {
    public function generateKeyForUser(Release $release, User $user)
    {
        $randomHash = substr(md5(uniqid(mt_rand(), true)), 0, 8);
        return ActivationKey::create([
            'key' => $randomHash,
            'user_id' => $user->id,
            'release_id' => $release->id
        ]);

        // Mail::send( ... )
        // etc
    }
}

class UserActivationKeyController {
    public function __construct(ActivationKeyGeneratorService $generator) {
        $this->generator = $generator;
    }
    public function create(User $user) {
        $this->generator->generateKeyForUser($user);
    }
}

Solution B: Put the logic inside a trait

trait ActivationKeyGenerator {
    public function generateKeyForUser(Release $release, User $user)
    {
        $randomHash = substr(md5(uniqid(mt_rand(), true)), 0, 8);
        return ActivationKey::create([
            'key' => $randomHash,
            'user_id' => $user->id,
            'release_id' => $release->id
        ]);

        // Mail::send( ... )
        // etc
    }
}

class UserActivationKeyController {
    use ActivationKeyGeneratorTrait;
    public function create(User $user) {
        $this->generateKeyForUser($user);
    }
}