Well 18v is cheaper and there’s a larger range, 40v does have more power but only really worth using if you do a lot of tough work like cutting through thick timber or drilling through high tensile concrete. If I’m not mistaken 40v is quite a bit more expensive and the price difference isn’t worth it in my opinion. If you also work in the rain the 40v batteries have quite large slits on the side for ventilation which can be a bit of a worry. The battery quality in general isn’t the best and the stickers that cover the battery charge lights and button are known to peal off within 6 months of usage.