In a recent book Richard Bradley conjected that henges were internalised structures, whereas stone circles were externalised. What he was saying was that from inside a hange, you can see little of the landscape around, whereas from inside a stone circle the landscape interacts with the stones, allowing them to highlight specific features and perhaps at specific times.
This of course makes the assumption that by the time of the building of the stone circles, that the tree line was sufficiently far away from the stones so as to allow this interaction to be realised.
