The 230V ones have a built-in transformer, so they tend to be larger and more expensive than 12V ones. However, 230V bulbs may be used in existing 230V fittings and wiring, without having to rewire for 12V. So 230V bulbs are much simpler to use in an existing installation. Also, should you ever change your mind and want incandescents back, the 230V is ready and waiting.
As far as I know, there is not much difference in efficiency. Maybe if you were using a *lot* of them, 12V bulbs with a single transformer might be a little more efficient than 230V bulbs with their built-in individual transformers.
If it is a new installation, I would use 12V bulbs and a single transformer - it will be cheaper in the long run and the wiring is mostly safer.
Remember though that the 12V bulbs will draw significant current (50W draws 5A, 20W draws 1.7A etc) so the wiring must be thick enough to cope, especially if you are going to parallel several bulbs on the same wires.
Hope this helps,
FoxyRick.