The atmosphere of Venus is just very thick. Also it contains many useful elements, C, O and H, which can be used to build basically anything if you have enough solar energy. The problem is the (comparatively small) amounts of other elements.
On Mars, metals are very abundant and easy to extract, and also minerals suitable for making glass or ceramic materials are abundant, but the raw materials for making food and organic materials, like plastics, are very scarce and expensive to concentrate.
On Venus, there are abundant resources for making organic materials and food (except for a few metallic bioelements required in small quantities, i.e. Fe, Zn, Mn, Cu, Mo, Co), but there are no resources for making metallic, vitreous or ceramic materials.
However, the materials that are missing on Venus are easier to transport from elsewhere, because they are required in smaller quantities and they are dense solids that occupy little volume. If not enough water would be found underground on Mars, that would be really difficult to transport from elsewhere.
I was under the (uneducated) impression that there was a fair amount of water ice locked up in asteroids that are fairly easy to redirect into a Mars capture orbit.
> The caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparatively thin layer about one meter thick on the north cap in the northern winter, while the south cap has a permanent dry ice cover about 8 m thick.