Materials end up being a personal preference and cost thing. Here’s my thoughts and experiences, but to each her own.
PUL- was unavailable locally when I started, so I never went there. I do own a few items now that are PUL, but I’m careful washing them and don’t dry them. Probably a conservative approach. You can get anything from a basic white or colored, to all sorts of prints. I believe you can also have fabric sent out to be laminated, and there is a product I’ve seen that you can do your own laminating at home, if you feel especially competent. No mater what the option, though, it’s expensive. I think $7 a yard is as cheap as I’ve seen it (0nline). It’s $9 a yard at the local Jo-anns for white and a few basic colors. I suppose with the right coupon, it would be worth some experimenting.
Nylon/ ripstop nylon/ “swishy” fabrics– I use these as my water resistant layer built inside of my diaper (which means I can use it even if it’s hideous!). In a lot of my designs, it’s completely enclosed. In the diaper below, you can see it some- it’s the yellow. I check how well a particular fabric may work based on how a little water poured on a sample surface reacts. If it soaks in readily, it wont work for that purpose. If it beads up and rolls around, even after rubbing it onto the surface of the fabric, that’ll do.
T-shirts (and sweatshirts)- Some of them do seem to wear out a lot more quickly than others. I think a partial blend with polyester increases durability a little, but decreases absorption a little, too. For inserts, though, I only use 100% cotton. I feel there it has a lot larger impact. Some people prefer not to use them, feeling they just aren’t as absorbent, and rave about flannel. I will admit, though, that high stress areas (where pinned and around small areas of sewn surface) deteriorate quickly with T-shirt fabric. I originally used a lot of T-shirts, and made some very cute diapers with it. I was then very disappointed that I had to cover up my cute, time consuming rears with a cover. Even though I had a water resistant layer, the moisture would wick around the edges and at the velcro seam in the front very quickly.
Flannel– My experience with flannel is that it wore out to quickly for my liking, and wasn’t as absorbent. Plus I didn’t have as ready a supply of re-usable flannel material. It wicks the moisture out readily, just as the T-shirt does.
Fleece- I love fleece! Maybe if I lived in a warmer climate, I’d change my mind. But it works well for me. I can also find it readily in re-usable forms (blankets, sweatshirts, pants, bathrobes), so I don’t have to buy something new. Plus, it has a little stretch, the edges don’t fray, and it doesn’t wick the moisture out from inside of the diaper. It pulls the wetness away from baby’s bottom and keeps it away from baby’s clothing. It does have it’s limitations, and it’s not waterproof. If I didn’t have a water resistant layer in my diapers, it would let the moisture dribble on through if the diaper isn’t absorbent enough. But even overnights, fleece is usually a sufficient cover for my diapers. It’s also a sufficient outer layer (if the rest of my diaper is constructed well) to go without a cover (although I still use a cover if I won’t be able to change as often).
Not all fleece is created equal, though. Fleece should be polyester, not cotton or cotton blend for these purposes. I also use both heavier and lighter weight fleece- heavier for the outside of a diaper or a cover, and lighter for the inside of my pocket against baby’s skin. If you find a particularly stretchy fleece, in my experience it is more suitable for the inside than the outside.
Terry cloth– new yardage or recycled towels or washcloths- some people love it for in soakers. I don’t, but we use old towels for other purposes that I don’t use an old sweatshirt for.
Wool, Acrylic, Rayon, etc. sweaters– can be made into covers, but don’t try to integrate them into your diaper itself. As a cover, they can be aired more and washed less, subjecting them to less wear and tear as well as less exposure to picking up lint in the wash. I tried and failed, so now I don’t use any- I prefer other fabrics.
Note- covers can be sewn out of re-usable sweaters or knit/crocheted new. Patterns are out there if you want to knit/crochet.