Having played as both Khajit and Argonian (current playthrough), I can say that both are good, depending on what you want to do with them. Khajit are nice because they have night vision, Argonians can breathe underwater and have the hitskin (sp?) ability, which allows for fast health regeneration once per day. Both my Khajit and my Argonian were Archer/assassin characters. As for the Thalmor embassy stone of Barenziah, if you take a horse into the courtyard, you should be able to ride it up over the wall (glitching) by the tree on the left hand side of the main building, that's how I got it on my first playthrough.