Your mechanic is full of bull plop. I have used both on my 255/55 18s with no issue. Your question would be better answered by what kind of diriving you do. If you are strictly country roads or around town with little snow then if you MUST buy something just get cables. Chances are with good tyres your truck would do just fine anyway. If you are looking at off pavement or deeper accumulations then chains might be a better option. Chains might last longer depending on how you maintains them, speed and conditions. Usually chains are quite abit more expensive.