Stone mulches create an environment often too hot for the plants. Plastics look unsightly. Wood chips are attractive and rot slowly, adding valuable organic matter to the soil. They keep the soil cool and moist, but mice living in the mulch in winter can chew the bark and kill your trees. Keep the wood chips a couple inches away from the trunk.
String trimmers are popular and easy to use.
They do a good job but can severely damage the tree when wielded by zealots. The nylon string can cut through the bark and an underlying conducting tissue called the phloem. If the phloem is severed, nutrients cannot flow from leaves to roots. The plant weakens and dies as the roots starve. The damage is similar to that caused by mice in winter.