We've had experience of every combination of floor/no floor, straw/no straw etc etc. Our soil is very heavy and poorly draining so mud is a big problem. We've got 8 or 9 arks some with built in wooden floors, some with loose wooden floors and some with no floor. I've found I need to play it by ear and see what works best for any particular ark in any particular location. Currently we have one that has no floor at all and no straw, but is very dry inside and houses the cleanest and driest pigs we've got. At the other end of the spectrum I had to move two sows yesterday despite having a wooden floor and straw because the whole set up had become waterlogged. The liquid mud outside had built up so that it rose over the wooden floor inside. Our boar and his two current ladies have a wooden floor but no straw. I got fed up of them taking it all outside as soon as I gave them a fresh bale. They seem to be happy and dry on the wooden floor.