Can anyone comment on whether one should rent a car or use public transit (train and bus)
If you don't mind driving a car then car rental is the best way to go if you want to see some of the remote parts of Scotland where public transport is limited. Bear in mind though that you can miss some of the best scenery if you need to concentrate on the road ahead.
You could hire a vehicle with driver but that doesn't come cheap.
If you are content to limit your travels to the main tourist routes then public transport is generally frequent and cheap.