I have received a pack of resin and hardener,and the only mixing information is by weight.

As far as I know I think it is still easier to mix by weight :p might be personal.

As Matthieu said, most materials (especially liquids) change volume with temperature, the resin and hardener may change volume at different rates so your mix ratio would be different at 17deg and 25deg. Maybe only marginal but ratio by weight is more accurate and will not change the ratio if working in different temperatures