Bigger Is Better
Outphase84 has it right - the higher the breathability number the better. It is easy to make something waterproof - a plastic bag is waterproof. It is also easy to make something breathable - a wool sweater is breathable. The trick is to make something both waterproof AND breathable. You chemistry types would love the fabric industry! It is pretty cool what they are able to do these days and we in the snowboard world are lucky to be on the cutting edge of the industry. Truly - we get some of the best stuff - straight from NASA to your bod!
To make a fabric both waterproof and breathable they use a combination of liquid coatings and laminate layers. The coatings I have already talked about - they create a barrier that does not allow water droplets (the liquid form of water) to soak into a garment. The laminate layers are key however because they are made of a a porous material that stop water droplets from getting through but allows water [I]vapor[I]to pass. Sweat, by the way, is water vapor, or steam, the gas form of water.
So the key is in the rating. It is hard, and therefore expensive, to make a stable laminate layer that has enough of these tiny holes (pores) to allow the garment to breath effectively. The more holes the better you breath. The more holes, the higher the rating.
Ok, I realize this is a totally over-simplified explanation of some of the most advanced fabrics on the planet but it should help with your basic understanding of the breathability side of the equation.
One thing to keep in mind is that breathability is only for moisture vapor. That is why what you layer with is so important. If you have sweat that builds up so you are dripping wet that sweat isn't going anywhere. You need to layer with wicking layers that draw the moisture away from your skin and will allow that moisture to evaporate through those little holes we just discussed. Anything that feels wet to the touch when you take it off is not a wicking layer.
Wicking layers spread the moisture out over a large surface area and will feel essentially dry to the touch even when they are saturated. Fabrics that wick are true performance fleece (no not the junk you buy at the mall), constructed hollow core fabrics and some treated natural fabrics but NOT COTTON.
Aside from leaving you feeling waterlogged any sweat that builds up will suck all of the heat out of you. The first rule of keeping warm is keeping dry. So ditch the cotton hoody for the slopes, layer with stuff that wicks and stick gear that breaths.