Fleece is made of petrochemicals and eventually will become microplastics in the ocean while wool is made of, well, wool, and eventually will be eaten by moths.
Fleece seems marginally lighter though not much more packable. I think part of the push for fleece was higher profit margins. Icebreaker is pretty great, but too expensive for my taste. Costco had some wool tops for $20, and Taiga makes stuff in Canada that’s quite nice.
I got a merino wool sweater from Duluth trading that is better than other loosely woven, poor yarn ones costing twice as much.
Oh sure ! I’ll jump in on this. Wool. No contest, not even close. Now wait, there are many different wools. I personally detest ye ole, itchy wool sweater. Ah, but warmth? I wear a cashmere sweater to bed on all fall/winter/spring canoe outings. A merino wool shirt or even long johns, for an early spring 200 to 500 miles? Outside entirely for April and May? Wool, Merino wool, lambs wool, cashmere, and for socks, there ain’t no contest, winter, spring, summer, fall,Bison down socks, or Bison down/yak, socks, bison down/merino wool socks. I paddle a canoe, in rapids I am on my knees, often for hours. I do not care how good your spray deck is, (Mine is wonderful.) or skirt, or what you are wearing, your knees and feet are soaked. What do you have to have on board, 2 quarts, 3 quarts, your feet or knees are soaked. Period. The entire day. Wool will keep you warm even if dripping wet. Only a diver’s wet suit can make the same claim.
Wool… But why wear, or compare the stinky nasty, itchy stuff. Spend a couple of bucks, Merino wool, cashmere, bison down. Fleece,I have few kind words to say about fleece. It works great if your trip is a day trip, or even an overnighter, a dry overnighter. Gonna spend a week or a month out? Use wool.
I spent part of 2 years working in SE Alaska with 150 inches of rain. We waded rivers all the time. If it wasn’t raining the brush was wet. I learned to really appreciate wool garments. I gave up on rain gear entirely and went of Filson tin cloth which could take the brush without ripping, and was not slippery if I fell on a steep slope. I started every morning even in July and August with a suit of Stanfield wool long underwear made in Canada.
When wielding my axe I only wear wool.
Wool is the only material I know of that can get wet and maintain a decent amount of its thermal properties. In many situations this is not trivial. It probably saved my life, or at least kept me out of the ER, once in my younger days.
But if it makes you itch then fleece wins. The only place I can have wool against my skin w/o the itch is a my feet. Even alpaca and merino doesn’t make it for a top without a layer between.
I think someone forgot about neoprene and the various fuzzy rubber types of clothing. 
Neoprene is fine around the water. On land it is impossibly hot and sweaty and clammy.
It’s been in the high 80s, low 90s here. Don’t even mention wool or fleece. 
In the mountains, frost and snow are common in any month. Years ago it was 107 in Reno for many days in a row. It was record setting. I called two friends and we hiked up to Bishop Pass in the Sierra. We had frost every night at 10,000 feet. Last year I was hiking near my house up by Carson Pass, on August 20 it was 23 degrees one morning.
Another thing about wool is that it is self-extinguishing when hit by sparks from fire. Synthetics tend to melt, and some (like polypro) will actually burn through and stick to your skin. During the years I worked construction in the winter I used to buy Army and Navy surplus wool pants and shirts and re-tailor them to fit me. Wore like iron and I was never cold, plus I never had to worry about welding sparks setting my clothes on fire. I wear wool socks year round – the only fiber that keeps its loft and cushioning when totally wet. I still have a lot of vintage wool stuff including my Woolrich Alaskan shirt-jac from the 1970’s, but the newer washable wools are an excellent upgrade. I have even paddled in cold water using dense 100% natural wool Dachstein knit gloves.