Thought I should respond to original post. <img src="/forums/images/graemlins/smile.gif" alt="" />

In a thin layer like 100wt fleece at 8.5oz per square yard your not going to get much weight savings with any choice. For a fair comparison though you have to think of what it adds to your total clothing system and/or sleeping system. I suggest a lightweight but looser knit wool sweater. It might weight a little more than the 100wt fleece, but in summer you might be able to eliminate a skin layer underneath, and when worn alone it should be just as cool as the 100wt fleece, and when worn with your wind layer or rain layer it should be just as warm and light as your 100wt fleece with a skin layer underneath it. Wool also has the advantage of thermoregulation by capturing the latent heat and sensible heat from your body moisture, especially when you need to add your wind layer over it, and then giving up that moisture when you get moving again, especially in sunshine, and want to take that wind layer off. It effectively acts as though it had a higher r-value while gaining moisture, and as though it had a lower r-value while losing moisture. It gains up to 35% without feeling wet, and can gain more than that and still provide insulation.

The other thing to consider for saving weight is to make sure you have fairly even coverage when you are wearing all your clothing at once for a particular trip, such as when its colder then normal and you are stopping to eat or rest. Especially in Spring/Summer/Fall it is normal to hike in just shorts but perhaps a sweater up top, but when you stop it is more effective to cover your legs and add a hat and mitts than to add another top layer, other than your wind layer. Wind layers are always good. When you are moving though, or when you are stopped in less cold conditions, the wind layer can be the first thing to take off even if its windy, to dry your insulation out. Doesn't really matter much what you add or remove though, whatever is conventient, but when you are wearing it all at once you should have fairly even coverage and a single wind layer top and bottom.

Example...

Say this is your system now:

Bottom:
boxers
4oz polypro
7.5oz 100wt fleece
hiking shorts

Top:
4oz polypro
8.5oz 100wt fleece
precip wind/rain jacket

Hat and mitts and socks important but left out for clarity.
This is pretty much summer fare, so its harder to tweek, but perhaps...

Bottom:
merino boxers
7.5oz 100wt fleece
4oz wind pants
hiking shorts

Top:
12.5oz loose knit merino sweater
4oz wind jacket ?
8oz rain layer ?

Anyhow, you may want more than this, but you get the idea. Personally I like to separate wind jacket and rain jacket, but I haven't tried the precip, and for a summer system like above I would be tempted to combine the two into one as you seem to have done.