Hi,
Bedding is designed to give comfort to cows therby increase the yield and maintain helathy status. Opitons mentioned both sand and mat are good both have there own pros and cons.
bedding could be decided on the pattern of animal housing if they are confined to one place for 24hrs on concrete then rubber mats are best , but if its loose house system then paved soil could be gud.
Any way sand is more expensive as u need to change after some duration and regularaly stir and aerate the bedding.
Happy dairying