Polyester fleece can melt and burn. Given their insulation value it is no wonder people have been known to back up to a fire and catch themselves on fire while being completely unaware of the problem. There is supposed to be a Nomex fleece available that doesn't. Wool chars but won't melt.

On the other hand polyester won't rot or get eaten by moths. If you are storing blankets long term in less than ideal conditions, like perhaps in an underground cache, especially in buggy Florida, there is a good chance your wool blankets end up with holes. Lost my favorite OD green Marine 'Wooli-pulli' to moths even though it was supposedly moth proof. Put it into a drawer in January and it was fine. Come next December it was shot.

Most poly blankets let the wind blow through. The average wool blanket is tighter. But the more expensive tighter woven poly fleece is almost as tight as your average wool and poly with a laminated wind barrier blocks wind better than any wool blanket.

Wool maintains some insulation value when wet but so does poly fleece. Wool holds about 30% of its weight in water and sometimes more. Even at the minimum value it takes a long time to dry. Poly, wrung dry and shook, holds less then 10% water and dries swiftly worn wet.

Wool tends to be expensive for quality but ex-military blankets of good quality can be pretty cheap. Fleece is cheap, used to be cheap fleece was pretty miserable stuff and quality was more expensive than wool, but prices have dropped. Now good quality fleece, tight, dense, high loft stuff that doesn't pill, is cheaper than even cheap wool. Laminated fleece are still pricey but but they should be coming down as they gain popularity.

Also wool is heavy, when it gets wet it is heavier still. Poly is relatively light and after you wring it out it stays light. Don't wring that wool or it will get pulled out of shape. I have a trapezoid shaped blanket at home because it wasn't laid out properly when it was drying.

Wool combines many rolls. Tightly woven it will shed a good amount of rain. Tight and dense it blocks wind pretty well. And, of course, it is pretty good insulation.

Poly fleece doesn't shed rain unless it is treated with a water repellent. Treated with a DWR it sheds rain well and it will even after a few washes. The looser weaves don't stop the wind. This can be an advantage.

Personally I have both wool blankets and poly blankets. Forced to pick only one I would go with the wool. But usually I pick fleece and include a lightweight nylon tarp that is a little bigger than the blanket because it gives me a lot of options. In moderate cold the fleece alone is great. Add rain and the tarp becomes a rain shelter and, perhaps, a ground cloth. In wind the tarp gets lowered on one edge and becomes a lean-to. Colder and windier I tighten up the tarp. If it gets warmer and it rains, like it does down here, the tarp works alone. If it gets a lot hotter the tarp keeps the sun off.

The kicker is that the fleece blanket, light nylon tarp, a few lengths of light line and a few stakes weigh less and are less bulky than the wool blanket. And even after being stored for a few years I can expect it to come out of storage without holes.