To the point of the author, the only difference between all the different types of T420s series systems is CPU and GPU. Why list all the other components in a list when they are all exactly the same?
Our organization bought several of these shortly after release. All of them developed a serious crack in the top of the enclosure around the trackpad (a bit right off-center) within a month. Some also have hairline cracks on the right side. The wireless drivers were also incredibly problematic upon release until they were updated.
If you are comparing a T series with a MacBook Air I can tell you have never actually touched or picked up a MacBook. You should really visit one of the stores and check them out.