These two breeds are both longwools but one is from Australia (Polwarth) and one is from the North of England (BFL). Many sheep are named from where they come from which can be a deciding factor for choosing a particular breed. Some animals eat different things depending on where they come from so they produce a different fiber.
Polwarth sheep are a blend of Merino sheep and a Lincoln sheep (more Merino than Lincoln), which have long ringlet curl-like locks. This combination makes for a fine wool that produces next to the skin softness. The staple is very lofty and elastic so it offers good drape for your finished garments. These are hearty sheep so they produce a durable fiber. The micron count ranges from 21-25. These sheep fare well in wet climates and do not get rot in their fleece. Polwarth fiber is a joy to spin, the long staple makes it great for beginners.
Blue Faced Leicester or BFL, has a wonderful luster and natural sheen that when you add silk to it makes for a heavenly spin. It absolutely loves to absorb dye and blended with other fibers gives life to duller fibers. It’s one of the most ubiquitous fibers out there, just google it and you can find it anywhere. It's similar to the breed of sheep from the movie Babe, the one difference is that it has a dark face but that same long nose. BFL is a great fiber for beginners, no need to card the fiber, you can pull straight from the length of top and away you go. At 24-27 microns, you can get a good grip on it and still enjoy the creamy softness.