Don't even bother. Two pieces are more versatile than one especially when it comes to changing weather and climates, or even just a chilly airplane cabin or lecture hall/ seminar room/ library where you cannot control the temperature. you want to sit there with your hooded soft shell? I'd rather just throw on my fleece jacket.

Weight saving will be minimal and space savings inconsequential unless your current fleece is a heavy Patagonia R4. That said, if you don't mind the price and the Yeti looks, I think the R4 is one of the best out there. It is wind and water-proof and super warm. Some say, they have worn it with a t-shirt at 32F. Then simply add a hat and your good.

There are some very nice high-tech watchcap style bonnets or some equally high-tech LoroPiano Stormfabric baseball caps, if you want something casual to go with the fleece.

Till