Microfleece is thinner so less bulk in your nappy. You can use polar fleece, it will just make your nappy a bit more bulky.
Sherpa Polar fleece?? Not sure what that is.. Is it from SL? Its not that Cotton Rich fleece that looks a bit like Sherpa is it? If its just polar fleece (100% polyester) you can still use it for your nappies. If its the cotton rich stuff which is 80% cotton it will be absorbant but not repel the water. I use the cotton rich on fitted nappies, its nice.

Any of the Polar fleece's at SL can be used for MCN, if you do 2 layers of it (or just a second layer in the wet zone to help keep the bulk down) it will give you a fairly good barrier as long as you boost it well. I made up a heap of pocket nappies like this when I was first starting with making nappies. But I like fitted nappies teamed with some woolies now so I use Microfleece for my inner(next to bub) then some nice pattern or cotton rich or flannel material as my outter & then what ever I happen to have as my absorbant material in the middle. So I have a 3rd layer with a long rectangle of absorbant layers sew on to that then my inner & outter layers. Add my snaps & im done!
there are lots of pics on my blog if you wanna see.