Opinions please! I've never used either. Thanks

Love it! They are not hard to work with, and they are very warm!

Just be sure to prewash the flannel. It tends to shrink, sometimes a lot.

I agree, prewash the flannel, it will shrink. As for the fleece, it's extremely warm! It is polyester, so it won't shrink. But it also stretches more than cotton or flannel, so just be careful not to pull on it when you are stitching. Also, with the fleece, it can help to pre-wash it, so that any stretching it did on the bolt has a chance to be corrected before you use it.

I have used both. Flannel shrinks so I wash and dry twice. Plus the lint. It also stretches a bit with sewing so I've had to adjust for that too. I have used fleece as batting several times. I like it. I've had no problems with it.

Be prepared to clean your machine often throughout your quilting as they both produce a LOT of lint.
Nice and warm and cuddly, though.

Some fleece has also shrunk - but this was from about four years ago -

How do I know this? Washed a blanket made from two different fleeces - after drying this blanket, it was wrinkled from the uneven rates of shrinkage.

As expected, such good advice!

I use flannel a lot for baby quilts. So cuddly. Just be sure to wash it. Fleece is just to stretchy for me. I don't like using it. I made a quilt for my soon to be SIL with fleece and way nearly bald when I finished. He loves it though.

I used flannel for a twin size quilt and didn't prewash. The book I got the pattern from suggested this. Didn't see much shrinkage.