Fleece or Down? Really, this is not much of a question to me. You need both. Perhaps debating the merits of a vest might be more appropriate.
To me, a vest is additional core insulation. It provides extra ventilation which is at odds with the insulation but does a good job under a rain jacket or parka. 'Corse you loose the extra ventilation that a vest provides. At night, these work fine, but in the morning, again, the extra ventilation can lead to some heat loss as you pack up your gear. At 12oz, a full down sweater (First Ascent or the like) will do a better job. It is OK to put something over your shoulders when you get up, though. Evenings, night and mornings is when I really need extra insulation. I term this "sleeping cloths," though, in a worst case sceanario, they get used otherwise.
Your arms (and legs) can get cool. Your body has a way of shutting off blood flow to your extremities. So, this does not really effect your overall safety. However, you will be more comfortable with your arms covered. I tried a down vest but was not very happy with it in the mornings. At night, it didn't much matter and I thought I had a winner. Iff you wear it during the day, it may get wet. In either material, you DON'T want it in the sleeping bag. So both being bad to use at night if they are wet, I think it is possible to ignore getting it wet.
Fleece is simply not as good an insulator as down. So, for summer, fleece works best because it is thinner. However, down compresses better…the bulk mostly a product of the two layer shell material. Fleece uses a single layer that is scratched out in a controlled fashion. So you have a single shell with insulation on both sides. If you use it during the day, it will get wet. So does down. Wet down is worse than wet fleece. But, if you will be using this as sleeping cloths, then it should NEVER get wet in the fist place. Hmmm…
This means that you will always be packing the vest, if you plan on sleeping in it. Using it when you stop at camp, at night and in the morning, means it pays in every case, though. While hiking, you need something different. Except in very cold (snow) or dry conditions, you simply cannot use it for both, unless you get in a pinch. If you have to take it off at night because you have a warm night, that's OK. You'll need it too often to not bring it. It also lets you use it as extra sleeping insulation. This means you can use a lighter sleeping bag. Basically, you save about half the weight in reduced sleeping bag thicknesses. The savings are somewhat less with fleece vs. down, due to the higher insulation qualities of down. The overall packability of down vs fleece means that when coupled with the lighter bag, you won't be adding extra bulk to the pack with a down vest (or very little with a fleece one.)
Other than a regular washing, down does not need any maintenence. Simply hanging it loosely, upside down between uses, works best.
Fleece does. After each trip it should be washed (easier with fleece than down) and brushed with a stiff hair brush. Otherwise it looses performance rapidly. The fibers will get matted and tangled loosing insulating value if you do not brush them out. DO NOT dry fleece on hot. The fibers can kink and mat. Rather dry it slowly like down. Even then, it is only good for a couple seasons of use before it will start failing. But, compared to a down vest, it is cheap. You can usually buy three or four for the price of one down item. Down will last a whole lot longer. As long as the shell material is intact, it should be good…10-15 years, easily. For long term use, down is highly recommended.
I believe in Skurka's book, he has a table of the relative merits of the two. I disagree that his sleeping cloths should be included in this matrix, though. Sleeping cloths will never get wet. Nor does he discus the relative comfort of having a seperate insulation for bed that can stay on you when you wake for pee time, and in the morning. And the weight savings of the sleeping system because of the extra insulation (vest or sweater) being included in the system. (The lack of a more systemic approach -related items, gear and techniques- is my only real criticism of his book, BTW.)
IFF you do not use it as sleeping cloths, or are traveling in wet conditions, fleece is better at holding insulating qualities when wet and it dries much easier than down.
So, in closing, I would recommend both. It depends on when, where and what conditions you expect to encounter. I bring both types of sweaters, but, in the ADK's I prefer long sleeves against all the bugs. Fleece for day use, it is better wet and ventilates a bit better. Down for camp and sleeping. Both when it is very cold…around 10F and I am hiking.
Fleece is better in wet conditions
