It would be more efficient just to redirect some comets. There is more then enough of everything floating around the inner solar system, and the Oort cloud has even more.
The composition of Mars' atmosphere is less important then getting the pressure right - at 1 atmosphere, humans don't need suits just respirators, and Earth-microbes will be more then happy to normalize things to suit them (they did it here, after all).
For Venus the big problem is water: Venus is dry. The reason it's in run-away greenhouse is because all that CO2 just will not precipitate into carbonate minerals, which to do so needs water. Again: something comets have plenty of.
The difference between the two in suitability though is the tidal lock: Mars has a day-night cycle. Venus has a day-night cycle but it's 116 Earth-days long - half the planet, if you got the atmosphere under control, would be in a freezing night for 3 and half months. You could settle it, but life would only really survive at the polls. While we can process a planetary atmosphere with plausible near-future technology and local resources, there's no known way we could "spin up" Venus (ironically a fully-tidally locked planet would have much more usable surface area as an oculus-world).
The amount of water available in comets is laughably inadequate to fill Venus's shortfall. But anyway its day length would make it very close to useless to try to terraform.
Shipping in enough hydrogen from (say) Neptune might be possible in principle using billions of automated, self-reproducing nuclear powered spacecraft.
If its atmosphere could be precipitated and the carbon freed of oxygen and somehow permanently protected from runaway combustion, the planet's low (2.64°) axial tilt and solar proximity might make a polar existence possible. But the overwhelming excess of oxygen would need to be removed or bound up in water. (The carbon might then be safely kept under water. Or, be crystallized out as diamond, which is hard to ignite.)
Quadrillions of aluminum foil balloons full of nitrogen bobbing in the stratosphere might suffice to bring temperature down.
Perhaps surprisingly, the present 3.5% of its atmosphere that is nitrogen is more than Earth's total.
As noted elsewhere, Venus's magnetic field is not much like Earth's, although it stretches almost to Earth's orbit, and might have crossed it in the past. (Such events might account for Venus's baleful reputation in to the oldest myths.)
The clouds (and presumably anything floating in them) ‘orbit’ the planet much faster than the planet rotates. It only takes about 4 days for the upper cloud decks to circle the planet.
From an energy-efficiency perspective, the difference is negligible. Getting out of the gravity well is by far the biggest hurdle. From there you might as well just give it a gentle push over to mars.
Why waste precious carbon that could be turned into life, industrial equipment, and fuel when energy from the sun is plentiful and otherwise wasted as it radiates into space?
The composition of Mars' atmosphere is less important then getting the pressure right - at 1 atmosphere, humans don't need suits just respirators, and Earth-microbes will be more then happy to normalize things to suit them (they did it here, after all).
For Venus the big problem is water: Venus is dry. The reason it's in run-away greenhouse is because all that CO2 just will not precipitate into carbonate minerals, which to do so needs water. Again: something comets have plenty of.
The difference between the two in suitability though is the tidal lock: Mars has a day-night cycle. Venus has a day-night cycle but it's 116 Earth-days long - half the planet, if you got the atmosphere under control, would be in a freezing night for 3 and half months. You could settle it, but life would only really survive at the polls. While we can process a planetary atmosphere with plausible near-future technology and local resources, there's no known way we could "spin up" Venus (ironically a fully-tidally locked planet would have much more usable surface area as an oculus-world).