Join Date: Oct 2011
Location: Illinois www.saddleonline.com
The grams (300 is fairly average) are more about the warmth than the denier, though a higher denier is typically indicative of a warmer, tougher blanket.
Since you live in a wet area that doesn't go a whole lot below 30F all that often, a waterproof (i.e. Make sure it says turnout blanket, not stable blanket) 600D, 300 gram medium weight blanket should work well for your mare.
If you are worried a mid-weight blanket won't be warm enough, but are concerned about her getting too hot, buy a sheet as well. You can always use the sheet on warmer days and layer the pair of them together for extra warmth on really cold nights.
"Riding: the art of keeping the horse between you and the ground."