Both are non-allergenic synthetic materials which replicate the feeling and warmth of a natural Down duvet. The difference between the two is how the synthetic fibre are constructed.
Microfibres are very fine fibres which are constructed in long ‘string-like’ strands. This creates an ultra-soft duvet.
Hollowfibres are constructed in ball clusters. Not quite as soft as microfibres but a perfect structure to hold pockets of air and create the required warmth.