Geno , the Powell lever lifts the Horsley lever slides .That may seem a simplistic answer but there is a whole heap of difference. The Powell lever is a lot stronger and gives more bite. One of the biggest problems is that the lever wears the axel [pivot pin] and can be a swine to get out and fitting a new one and filing it up is not so easy.
When the Horsley levers wear they are a nightmare to get right.But you have to accept at there age and that nobody ever thought that they would still be in use .Lets face it most of them were getting on when they first saw a Model T Ford and how many of those are still in daily use?