Never mind my little semi-informative rant here, but as an astronomy buff...
One tiny little drawback to either planet's terraforming: lack of a detectable magnetic field. I'm sure you've heard of something we have on good ol' Terra called a Van Allen Radiation Belt? It's effectively a magnetic trap in the planetary magfield, where radiation and charged particles are caught. The interesting thing about it is it serves to deflect cosmic radiation being emitted by the sun.
Another thing, from a geological standpoint: magnetic fields are caused by convecting liquid iron in a planet's core. The 'flip-flop' phenomenon we're expecting on Earth is a result of a shift in the current flows. Now, Mars is effectively a dead planet: the core's probably still hot, but not to the free-flowing degree of Earth. Venus lacks a magnetic field, as well, but I've not heard any reason why it doesn't. Can't use the 'cool core' explanation because it's too damned hot there.

End result? Unless you can somehow jump the core of a planet up to around the surface temperature of the sun, and get that liquidized iron flowing about, you're not going to get a magnetic field to deflect the solar wind from stripping away any non-contained atmosphere you generate on it, and no radiation belts to provide an additional defense against cosmic rays. That means an atmosphere-tight colony design: underground and/or 'bubble cities' for the colony proper.
IMO, we need to leave Venus alone for a good bit, until we find some method of peeling off that thick atmosphere so it can cool down a bit. Maybe some kind of orbital shade to block the sunlight coming in on it would be a good start...