Full face. Safer. Plus required for open top cobras, even with full windscreen, in Oz.
I ride a motorbike, so recently when I needed to refresh my helmet I considered options. An SA rated car helmet is lighter and has a wider eye opening for greater visibility. Also optionally has the HANS knobs attached.
However the new motorbike helmet I purchased was an upgraded Shoei (what I normally ride in) but this was HEAPS lighter! It also included as standard the fog free visor, PLUS a built in sunglasses pop-down thingy like the jet pilots use - which is ultra cool! It means you can add and remove sunglasses with a flick of a knob on the side of your head! No need for uv visors, or shade challenges like Peter faced!
I think it is only slightly heavier than the equivalent car helmet. But it doesn't have HANS anchors.
As Bill said, make sure you go somewhere that know what they're selling. For example, Arai's don't fit my head whereas Shoei are great - for me!
Oh, and my Shoei fits my eye glasses underneath as well - a more recent requirement
The Shoei GT-Air is pretty expensive - about $1k AUD. But hey, I like my head in its current shape.