Flannel and fleece are two of the most popular cold-weather fabrics. They’re found in everything from mittens to bedsheets.
Fleece is an incredibly warm fabric that traps heat and doesn’t allow it to pass through the other side. Flannel is also warm, but it has easy breathability.
Flannel is a brushed type of knit that’s traditionally plaid. It’s also known for its softness.

Durability
Both flannel and fleece are durable materials, lasting a long time when properly cared for. Flannel is a natural fabric that can be made out of cotton or wool. Fleece, on the other hand, is typically made out of polyester. Polyester production is known to leave a huge environmental impact, including polluting water and land. Some brands of fleece do offer recycled or organic options, which is a better option for the environment.
Both flannel and fleece can be used to make cozy pajamas, all types of winter clothing, blankets, and more. However, they have different advantages when it comes to durability. Fleece is much warmer than flannel because it traps heat well. But it also doesn't regulate your temperature as well, which can cause you to overheat. Flannel, on the other hand, is breathable and allows excess heat to escape. This makes it easier to stay comfortable all day long. However, flannel does tend to unravel faster than fleece because of its loose weave.

Softness
When making a blanket, you will want the fabric to be soft so that it will feel cozy against your skin. Fleece is generally softer than flannel because it uses natural fibers rather than synthetic materials such as polyester or plastic. However, it may have a slightly plasticky feel to it that some people are not comfortable having next to their skin.
The flannel, on the other hand, is made with cotton and wool, which are known for being very soft. It also tends to get softer with every wash, which is another huge advantage over fleece.
Both fabrics can make great blankets, but it will come down to what you prefer and what your needs are. You may find that you need a warmer material for cold weather or you may only want a light blanket to use during the summer. Whatever you decide, both of these materials will give you the warmth and comfort that you need.

Warmth
Flannel is a great choice for winter sheets because it keeps you warm but doesn’t overheat your bed or body. It is made of cotton woven together and then brushed on one side to create a soft nap that traditionally features a plaid pattern. It is a very breathable fabric and can actually get softer over time. However, it may shrink if you wash it too often.
Fleece is typically softer than flannel because it is made from polyester which has a very fuzzy surface. However, it does not breathe as well as flannel and can trap heat in your bed, making you sweaty throughout the night. Additionally, fleece is made from plastic and petroleum products, which aren’t good for the environment. Flannel is generally a better choice for the environment because it’s made from natural fibers like cotton. However, you’ll want to ensure that the flannel you choose is made from organic cotton. If not, it will require massive amounts of water to grow and dye, which also uses chemicals that aren’t great for the planet.

Style
Flannel is a classic for the fall and winter, especially in the form of shirts or pajamas. The Flannel Fleece fabric feels smooth against the skin and warm to the touch. It also layers well under a jacket or shacket in cooler weather. It comes in a variety of styles from slim, athletic fits like Pladra's Leon to classic grandpa-style flannels like KUIU's HW Plaid. However, a tight flannel can bind the shoulders, so opt for a slightly tailored fit to prevent this issue.
Both flannel and fleece can be machine washed, but you should avoid adding anything made from a different material to the wash because it can stick to the fibers and cause a mess. You should also make sure to use cold water, as hot water can cause the fabrics to shrink. Fleece also sheds microfibers during washing, which can end up in the water system where fish consume them and then humans can ingest them as well.