Default IPR uses CPU, which is also very fast on it and supports more features.
IPR supports
interactive move of cameras and camera settings,
Light and light moving and settings changes,
hdri and dome light rotation and exposures, Sun and sky changes
object moves.
material color and channel activation changes and certain settings changes inside the channel(spec glossy settings etc).
Current limitations
Due a code part of the V-Ray sdk we yet have to use (UVWswitch) to run V-Ray on c4d, which is not supporting all things to be interactive.
There are some current IPR preview limitations, that need yet a stop/close and restart of the IPR window to update or render correct:
bitmaps or mapping changes wont update interactive- for that restart the IPR.
instances/render instances are not yet supported, vray proxies are supported, you might want to use proxies for that
some special objects like V-Ray clipper are not yet supported, this will come at a later point in service packs.