Fleece vs. Down (or both)

Down without question. The fleece is good for day hikes when you keep moving. But unless it is an unusually mild winter where you live, down is the better option for overnight.

As an off-trail, fisherman, dirt bag, never been a big fan of down's thin puffy sleeves in the backcountry (too easy to rip), but a down vest has a place. For me, a sophisticated layering system is much preferred. Assuming not winter camping, (down mandatory in winter), but for a regular, late spring/summer/early fall backpacking season Sierra, suggest:
Light, long sleeve shirt - when hiking
Fleece layer - always ready - lives on the very top of my backpack
Good, light, down vest...(Like the WM Flite series).
Waterproof (2.5 GTX?) outer shell layer w/ hood...
Maybe a pair of finger-less wool gloves and a wool balaclava.

Thus both.

I'm not exactly sure what seasons you want to be out in. I'm also not sure what sort of fleece you have, whether a heavy jacket type, a fleece shirt, or somewhere in between. Generally I would go along with what trinity said.

In cold weather, I carry polypro base layers, a fleece shirt, a puffy, and a shell jacket. With this combo I'm pretty good down to about 20, or just a bit lower, in camp. If I get cold I can wrap in my sleeping bag. On the trail I can mix and match my layers based on temp, sun, wind, etc.

It is your call. Carry what gives you confidence or experiment with less and build confidence in a new set up.

My approach is quite similar.  I carry a Patagonia R1 hoodie for warmth on the go, and a GoLite  UL down jacket for use in camp.  Worn together, they're good to about 15F.  I haven't found conditions yet I'd be willing to forego the R1 entirely in favor of down.

Just got back from a long day of cross-country skiing. 20-25 degrees. Sunny skies, no wind. I wore a Patagonia fleece vest over a smart wool base layer and synthetic shirt and a wool baseball hat. That was plenty warm when skiing in and up. At lunch, a down jacket came on along with a wool hat. Skiing out I wore a gore-tex shell over my fleece vest but the down jacket was hardly necessary. I was very comfortable the whole time.  I believe the same clothing  system would be comfortable in much colder weather except I might wear the shell and wool hat skiing in.

If I'm choosing between the two, I choose down. In summer, that is my only insulation layer. I use a long sleeve synthetic to help keep the mosquitoes at bay and a down jacket to throw on at camp. In winter, I often bring the same but have a rain shell to go over the top and at camp, I swap out to a merino wool base layer. I rarely bring a synthetic fleece layer unless I know it will reach single digits.

PCT journals are a great resource for what particular gear was successfully carried on their thru hikes. Nice post! Probably the most thorough list of what is available out there today that works well. However, long distance hikers are a slightly different breed of hiker than most...sure, most opt for the lightest, smallest, and best quality gear available. (Thru-hikers constantly talk and compare their choices...well-tested gear is recommended and opinions are always shared - that which fails is also noted too.) Gear is always a major topic on the trail...so is food!

You should realize that most of these hikers never get more than a quarter mile off any trail... carry the minimum...all about the miles...no frills...no fishing (and no food). Regular backpacker comforts and especially off-trail gear may be slightly beefier.

Ansel PCT, 1992