Seems that b is quite a good move. It rescues several points, allows for another 1 point gote play, and when the other side plays the sente, you answer it, have lost 1 point, but gained more by having played b. In other words: b is just as a is a 1 point sente, but it is "more sente" as the followup to that sente move is more points for the opponent if ignored (2 points vs 1 if I'm correct)
Remember, both and B are "reverse sente" for White. Since Black won't answer in any event, it doesn't really matter which one is "more sente" for B. (This is an important general point - one sente move may threaten to kill a group, and another just to reduce 5 more points, but if they're both 1pt sente, it doesn't matter which one B gets to play.) The point is that gains 1pt in reverse sente (F9) whereas B gains 1pt in reverse sente (D5) and still gives W a chance to create 1pt with the double-gote follow-up ().
Bill: b is 1.5 pt. reverse sente, the largest play on the board.