Terraforming is not necessary for colonization, but:
Venus requires importing about 40 quadrillion tons of hydrogen to lock up enough of the excess carbon and oxygen in its atmosphere in the form of water and biomass before its atmosphere can be thinned down enough to make the surface easily survivable.
Mars may have enough trapped volatiles to allow walking around on the surface in an oxygen mask, just by warming the planet up. A low-pressure breathable oxygen atmosphere could be built using oxygen sourced from rocks, with a byproduct being enormous quantities of metals and silicon.
Colonizing anywhere is going to involve moving large amounts of material, and ideally you'll want to get multiple trips out of the spacecraft used for the job.
Landing large amounts of mass on Venus requires heat shields and parachutes, and enormous air-deployed balloons if you want to avoid the surface.
Venus has nearly as deep a gravity well as Earth, and a much denser atmosphere. Launching spacecraft from Venus requires a multi-staged launch vehicle just like you'd need on Earth, except it needs to launch from a balloon floating in the atmosphere. A fully reusable Venus transport is basically impossible with any available technology.
Landing large amounts of mass on Mars requires heat shields and rocket propulsion. Small payloads can get some use from parachutes.
Launching from Mars is far easier than launching from Earth or Venus. A spacecraft that can launch from LEO and land on Mars can easily have the propulsion capabilities needed to launch back to Earth for another mission, if it can refuel on the surface. And Mars has the raw materials needed to produce propellants.
Venus would require buoyant habitats drifting through skies filled with concentrated sulfuric acid mist. Surface robotics would have to be built to tolerate conditions where common solders melt, aluminum alloys soften, and most plastics evaporate.
Mars requires little more than a pressure vessel and the life support you'll need anywhere. There'll probably be active temperature control, but it would be feasible to keep things survivable passively. In terms of temperature and radiation, it's a better environment than LEO. Mining equipment can use much the same materials and designs as it does on Earth...the big difference is that you'll probably run a coolant loop through a radiator for a lot of things that would use air cooling on Earth.
The only things easily acquired on Venus are components of its atmosphere: CO2, N2, SO2, H2SO4. The surface appears to be entirely volcanic, basalt as far as the eye can see (about 3 km under surface conditions).
Mars has a lot of basalt too, but it has had enough tectonic activity to give it a more Earthlike variety of igneous and metamorphic rocks, and a long enough history of liquid water to form all sorts of interesting concentrated minerals. There's clays, useful salts...one of our rovers got stuck in a patch of powder that appeared to be iron sulfate, and there's going to be similar deposits of copper minerals and other stuff you're going to need to actually build anything.