I sell both fleece and wool soakers at my Etsy shop, Winkydinks, and get quite a few questions about which is better. In my opinion, neither are better, they both are SUPER! Each has its pros and cons. I thought I’d share my opinions, in the hopes it helps someone else!
The way a soaker works is that it absorbs/repels liquid from the prefold/fitted/flat diaper before it leaks to the outer garment. This is compared to a PUL cover which is waterproof with elastic to hold it tight to the body/legs to prevent leaks.
Wool can hold wetness up to 30% of its dry weight, while still remaining dry to the touch. It contains natural lanolin, which acts as a moisture repellant and has anti-bacterial properties (which is why you want to wash it with a wash containing lanolin). It is also completely breathable, helping keep baby’s skin healthy while not getting too warm. Fleece is a synthetic material that has water repellant properties. Unlike wool, it does not absorb any moisture, but acts more as a barrier between the diaper and the outer clothing. A double layer of fleece in the wetzone (i.e. crotch) of the soaker is popular with fleece because it adds another layer for moisture to seep through.
The success of a soaker depends on how heavy your wetter is, how often you change, how absorbent your diaper is, the cover itself, etc. Fleece and wool will both compression wick. That means that the outer garment may feel damp to the touch, especially if the child is sitting or laying in one spot for long periods (car seat, crib). This is something you get used to, especially after having used waterPROOF covers. I personally have never had leaks to the point where the clothes are wet puddles.
-Wool will absorb extra liquid, fleece repels.
-Both are breathable (some people argue wool is more so).
-Wool is a natural fiber.
-Fleece is inexpensive compared to wool.
-Fleece can be machine laundered with diapers or regular laundry, wool has to be hand washed with special wash containing lanolin and laid flat to dry (which can take 2-3 days). On the flip side, fleece has to be washed often (I use mine a day or two, however some people wash them after each use) whereas you can air wool out and re-use it for several weeks between washings.
-Wool is generally more stretchy than fleece (good for night time diapers). The bonus here is that it might last longer before you need the next size.
-Depending on the thickness, wool is sometimes bulkier under clothing.
-Fleece comes in many prints and colors, wool is available in many yarn shades and can be knit or crocheted. Recycled wool (i.e. sweaters) offers some nice patterns/prints as well.
My personal choice:
I love wool for night time. A heavy weight wool will have no leaks (in my experience). I also like wool at night because its more stretchy than fleece and I can get it over a huge night time bum more easily. I personally like fleece covers for daytime because they are so easy to care for. I use prefolds or fitteds under a single layer fleece cover and have no leaks. To me, fleece is more "grab and go" and I don't worry about ruining it, but I also love the properties of wool – therefore my stash is about 50% wool AND 50% fleece!