We have floors in all ours, and after the rainy winter last year - you know the one that went from January 2012 right through to December 2012 ! - the pigs were very grateful we are on clay.
The short answer is that there is no single right answer - it depends on lots of factors the two most important you have already identified as dryness of soil and whether you are keeping all year round.
But if you don't want the expense of adding a floor, use pallets - free from any industrial estate. Just take the planks from one to nail/screw over the gaps in another - called castle boading. The pigs don't mind sleeping on this, just add a bit of straw. And you don't need to have a "fitted carpet" of pallets. 4-6 placed in the centre of an ark will provide a good sleeping platform, and the pigs will fill the rest up with mud/straw to level it out inside. If you need to move an ark, just leave the pallets behind and start again - ours then dig them back out for you over the next few months and we use them as fire wood.