Would be nice to have both options but if one has to choose one - then definitely cut objects have to become invisible to camera but still cast gi and shadows (like clipping plane in 3ds max cameras) - would be even better if you could implement a...
To show the atmosphere of a space in a section/floorplan we need hidden geometry to be respected in light calculations. This would be a great addition to Revit.
An indication of poly count (possibly even calling out certain layers/objects), texture sizes, light maps, etc would be super useful in looking for areas to increase model performance.
The possibility of doing section planes while preserving the hidden geometry for light calculations would be an amazing feature as none of the CAD programs respect that rule.