Paul is one of the greatest. You'll probably never go wrong reading his stuff and following his advice.

The difference is in the amount of data being accessed. The full scan of all pages gives you a more complete picture, while putting a much higher cost on the server. The limited scan gives you a less complete picture at a much lower cost. More often than not, LIMITED is adequate, if imperfect.

On a side-note, fragmentation is seldom a major issue for people. Usually spending time ensuring more accurate statistics works out better than attempting to eliminate fragmentation.