mountain_madness wrote an answer about Mountain Hardwear Jovian Jacket - Men's on January 10, 2012
Short answer: 1. (you need to know the definition for Hydrostatic Head) and you can compare the numerical values for an "on paper" perspective. Both are guaranteed waterproof.
2. They are both a similar concept looking at how breathable they are. (This is the air permeable aspect).
There are arguments and discrepancies with both answers listed here: rum is partially correct in stating Proshell has a hydrostatic head of 28,000mm, though this is the minimum standard required for a Proshell garment, it is fair to say 28,000+. Many garments on the market that use Proshell will be between 30,000 and 40,000mm. How breathable a garment is also depends on the face fabric used, so mvtr numbers should only be used as a rough guideline. Typically, the heavy, more durable face fabrics will be less breathable than the lighter due to the fact a greater surface area of the membrane is covered by thicker solid fibres which do not absorb moisture (this applies to nylon hardshells, not polyester used in soft shells).
It should also probably be pointed out that Neoshell and Dry Q are not the first air permeable membranes on the market, nor are they new this season. eVent is air permable, as is the Gore's Starlight. Essentially, eVent is based on the original design for the Gore-Tex membrane, which is expanded PTFE. The main difference is that eVent is expanded further, which allows faster transfer of moisture through air permeability, but reduces durability, and must be cared for more to retain optimum performance. Neoshell, as with eVent do not belong to Rab, they are on the open market for manufacturers, and yes, is 99% windproof according to Polartech.
All in all, this is all irrelevant. Anything with a hydrostatic head of over 4,000mm which has taped seams is classed as waterproof, and there are few places on the planet people will experience rain with enough force to equal the pressure of 28,000mm. So both will keep out the elements if you are an enthusiast (as opposed to a polar explorer say).
An air permeable garment will vent much faster than one that isn't due to the fact it doesn't require such a difference in internal and external humidity/pressure to shift moisture (like osmosis - if you have ever done that experiment with potatoes). However, I would not like to state the durability.
Leading to the final point of the face fabrics - with the heavier being less breathable. This depends on the ability to shed water, a fabric that does not repel the water and instead wets out will be much less breathable, so keep it proofed! And at the end of the day, lifetime is important. If it isn't very durable, the membrane will break down or become damaged much faster, and so none of the above will be relevant for very long!