I travel to Phuket 5-6 times a year and always rent a car. So far haven't had any issues. Even getting used to driving on the other side of the road, I haven't found it difficult to drive in Phuket. Due to the nature of the roads, you're always driving pretty slowly so have time to react to any obstacles. I would feel trapped without a car.
A scooter, on the other had, I would never rent. Death trap!
Where are you finding such a good deal on a rental? 4300 THB for 7 days is amazing!