They both have terraforming potential; just different problems to overcome. Over the relatively short term, Mars looks closer to falling within what technology and industry may be able to handle.
Venus has a very weak magnetic field induced by the solar wind interacting with its atmosphere (which strips lighter elements like hydrogen in the process). It has no intrinsic magnetic field. Mars has regional magnetic fields locked into segments of its crust left over from when it did have an intrinsic field. Either way, a magnetic field isn't necessary to block solar radiation; a fairly thick atmosphere with an ozone layer has that covered. Before Earth developed an ozone layer it looks like land got too much UV for much of anything to handle, but the oceans were okay.
For long term atmospheric stability over multiple billions of years, a planetary mass object should have at least 20% of Earth's mass, although it may take 30% to be fully stable. Mars, at 10.7% could hold an Earth like atmosphere for a "mere" hundreds of millions of years. Note that hundreds of millions of years is comparable to the Phanerozoic Eon which covers the entire existence of multi-cellular animals, and is also comparable to the expected time before Earth unavoidably goes into a runaway greenhouse effect.
You still have to get several exagrams (Eg) of atmospheric materials from somewhere though, and maintain a much smaller replenishment program if you want Mars to stay habitable for more that several hundred million years.
To precipitate out Venus' atmosphere, you'd need a few hundred zettagrams (Zg) of calcium and/or magnesium to react with the carbon dioxide to form calcium carbonates and/or magnesium carbonates. You'd also need around a hundred or so Zg of hydrogen as Venus is almost completely lacking in that. Any biological processing has no chance of going anywhere without the hydrogen. The solar wind will, of course, slowly strip hydrogen away, so you'd need to maintain a replenishment program for that, too. And then there's that pesky runaway greenhouse forcing from being that close to the sun.
So, in short, terraforming Venus looks to require ~100,000 times as much material as Mars, but can get potentially be made much more similar to Earth.