If you look at the photos of both products (new and old fluid goggles; the old ones are on the bottom of the page)
http://www.liquivision.ca/fluidgoggles.html
You can see the difference visually. The Classic goggles have a lower profile, are smaller, but are less adjustable -- the lens power can't be adjusted, and the spacing between the lenses fluctuates, sometimes causing occasional double vision. The old goggles also offer virtually no thermal insulation against cold water.
The new goggles are bigger, and cover more of the face, and hold more fluid. This offers MUCH more thermal insulation against cold water. The lenses, once adjusted, can never move around, reducing the occurrence of double vision. The lens distance from the eye can be changed, allowing the power to be fine tuned for everyone's vision. Further, by moving the lenses around in the goggle, you can select where you want your field of vision. For example if you are diving on a line and want to see 'up' very far (to see the bottom plate), then you can move and refix the lenses in the upper field of view. This is not possible with the old style goggles.
However, some people who want maximum hydrodynamics still prefer the older style.
Because the Nirvana-2 goggles require more fluid to fill, I recommend mixing your own saline with purified water and 1% sea salt. This costs virtually nothing (and is much cheaper than using contact lens solution). Using ocean water works as well, though the comfort of the ocean water on the eyes depends strongly on which ocean you are diving in (salinity).
