Early this year I bought into the Contax G system primarily for the fast sharp 21mm lens, and it has not disappointed. Now I have all but the 16mm and the zoom, and they're all outstanding lenses, a truly great series! My favorite focal lengths are 35/40 and 28, so the two Contax lenses I use most are the 35 and 28mm. I cannot dispute those who say the 28 and 45 are the two best of these, and though I've heard others say the 35 is the least of them, it's still a beautiful lens in my experience. The 45 is actually 46.9mm, and since I'm more accustomed to 40, the 45 (like 50) seems "long" to me. 🙂
As to the 28 and the 21, their angles of view are very different. The 28 is a widely useful carry lens for me, while the 21 requires shifting mental gears to "superwide mode", and is harder to use, more specialized. Wide enough that I consciously try to avoid common superwide cliches, playing to the wideness as a subject. Of course it's fun to explore the possiblities too... 😉
Here are three 21mm Biogon shots... Followed by two 28mm Biogon shots. Hope these might aid decision... 🙂