The question as to which is best depends entirely on where, when and how you plan to use it. If you need and are willing to pay for mobility, then you should buy a laptop or a tablet. If you plan to use it only at home and want extra power and a big screen then use a desktop. If you want mobility, then buy a laptop or a tablet. The choice between those two comes down to personal preference with regards to keyboard, mouse etc vs a touch screen.