The biggest difference between hollow fibre and microfibre is the structure. The fine fibres of microfibre are tightly woven fabric whereas hollow fibre does have spaces between its fibres.
Both materials will be a great option as they are cosy, warm and won’t cause problems for allergy sufferers.
If you would like a slightly more luxurious product that feels like feather, we would recommend microfibre. If this is not what you are looking for and budget is the main focus, we would recommend hollow fibre.