I didn't vote, cuz I use both shavings and straw/hay too. Shavings in the bottom, either straw or hay on top. I usually put alot of (usually) hay in the cage when momma starts her nesting dance. she'll pick it up and 'dance' around with it, and make the nest. Also somthing for her and the kits to nibble on.
I like to use wheat straw-not other. Just had to say I love watching them filling their mouths with it and carry it into the nest box. After they have all they want they pull breast/stomach fir to line it with.
The most amazing thing is a young rabbit that is raised in complete isolation from all others of her kind (except for that one night stand) builds the same nest for her young--no lessons--no "Nest Building For Dummies" books in her library (I checked)!
Makes you wonder how this Just Happens All By Chance.