I found an art supply site with an article about choosing brushes, which says that synthetics are a better choice when using an acrylic medium (which Future is), because the pH of acrylics is opposite to that of natural hair and will damage it over time.That was news to me. Obviously that's a tip about preserving the life of your brushes, not the actual application of Future, but it's good to know when you're going to plunk down $$ for brushes considering that all-natural hair brushes tend to be more expensive than blends or synthetics.
Personally I don't handbrush it that often - I prefer to airbrush Future, but there are those times when handbrushing is more practical. I don't have any brushes that are completely natural hair - mine are all synthetics or blends. I tend to prefer the blends.
Because of Future's self-leveling properties, I'm not sure the material the brush is made of is all that important when you're concerned for instance about brush marks. Swanny's article on Future says that that if handbrushing, the brush should be soft and free of dust particles and to use slow continuous strokes to avoid causing bubbles (definitely true). Let the Future fully dry before going over the same area again.