score:3
The bus is always cheaper but the train will be quicker.
It won't be hard to find either, personally I would take a specialist tour which would take you right up to Stonehenge itself, rather than having to go to Salisbury and then organise a taxi or local bus.
For trains from either Bristol or Bath check out: http://www.firstgreatwestern.co.uk/
for National Express Coaches: http://coach.nationalexpress.com/
But for specialist tours from Bath/Bristol http://www.scarpertours.com/tour/stonehenge
Hope that helps
Edit:
You might want to check this too: http://www.tripadvisor.co.uk/ShowTopic-g186370-i379-k582873-Bath_to_Stonehenge_by_public_transit-Bath_Somerset_England.html