Which style to choose. Opinions vary greatly as to what is the best style for bushwalking. Some people use light footwear for the heaviest walking and have no problems while most bushwalkers use solid boots in the same situation, the choice is up to the individual. For day and overnight walking on easy terrain and tracks e.g. Lamington, the lightest boots should be suitable. Runners can often be used but they give little ankle support, which is one of the advantages of boots.
Leather is definitely more waterproof but often that is unimportant when using lightweight footwear.
Most synthetics do breathe more and perform better in sweaty environments like the tropics. Sandshoes are good for creek walks and rock hopping and generally provide more grip. This support will mean that the ankle will become less tired, and hence improve enjoyment. As a side benefit, a less tired ankle can help to reduce the severity of injury if you do fall over. Also, even very light boots have a reasonable tread pattern which grips better than runners that have been designed for playing sport, not for bush tracks.