The clamp on style which fit onto the existing diff pan are good at protecting the thin pan from damage and are the least expensive style of bolt on guard, though after using one for a while my diff pan cracked round the weld. Some people have had these come off the pan, though I never had this problem and the ones I had were pretty cheap at about £15 each new.
If you have access to a welder then a potentially cheaper option is to fit a pipe/cylinder end cap. This is what I've just done, partly due to the existing one being cracked. Its a bit of work and requires fabrication, though the cost of the ends caps were just £10 each.
