I usually don't mention Portal 2 until they'#ve play the original. Avoids the "I don't have time for both right so is it worth it?" thought! Mention Portal 2 once Portal has been played and they want more now...
There are various bits of the commentary when they discuss previous versions of levels and why they changed things after the play-testing sessions. A lot more effort seems to have been expended getting things as close to "just right" as possible than you might first think.
I still feel Portal 1 works very well as a kind of introduction to the much bigger (and in most ways better) Portal 2. I also usually suggest people play the first one, especially if they're not that into FPS games, because it's less of an investment and focuses a bit more on just the puzzles.
There are various bits of the commentary when they discuss previous versions of levels and why they changed things after the play-testing sessions. A lot more effort seems to have been expended getting things as close to "just right" as possible than you might first think.