It will be a tough call on anyone who is left out.
Bichel:
Positives: Bichel's 2003 has been an improvement on past seasons, taking 26 wickets and averaging 29 with the ball. His batting has been especially helpful in the lower order, in fact he is averaging 28 for the year 2003 which makes him a bowling allrounder in every statistical sense. He also has the home-ground advantage and is in wonderful first class form with 10 wickets last week.
Negatives: He won't blast a batsman out, and therefore while his bowling is reliable in most cases, he isn't a new-ball bowler because he lacks the pace. If Australia wish to capitalise on the grassy wicket and get some early wickets, perhaps Bichel will be overlooked because of a lack of pace.
Williams:
Positives: He performed quite well in his first test match, and has pace to burn. Of all three applicants for the two berths, Williams is one player who will revel in the conditions if he gets it right because he can bowl straight to the right handers and move it away off the pitch. He can get steep bounce much like Gillespie, and can blast batsmen off the ground if he is on song. He is more experienced than Bracken, but not by much.
Negatives: He sometimes lacks control, and could be targeted by India's top order if India are able to handle the conditions. Williams also tends to bowl well in bursts, which means that if his first spell is not up to scratch, he may be letting loose one end in a session that needs to be tight. The other two candidates also have had success against the top-line batsmen. While Williams produced some excellent ODI performances in India, Bichel is a proven performer in dismissing the danger men, and Bracken's dominance of batsmen like Sehwag etc may prove costly to Williams.
Nathan Bracken
Positives: He is a left-armer who has had success against India, in India. His rise into Test status is interesting because it comes on the back of decent first class form, but importantly a dominance of India's most lethal batsmen in their home territory. Bracken's accuracy combined with his swing is deadly on a pitch like the Gabba, especially with the grass. His angle approaching a right hander means swing away to the slips and also the potential to trap the right handed batsman in front opening up LBWs or caught & bowled. Bracken's success against India is his main positive.
Negatives Youth, inexperience, and competing with the two bowlers who have proven themselves capable of facing tough opposition. Williams has already made his debut, so that extra experience may help. Bichel meanwhile has plenty of experience, can bat, and is bowling at a better level than that of Brett Lee.
Bracken's accuracy can be shot to pieces sometimes, and this may work against him - if he loses control he tends to spray the ball around everywhere. Hopefully this won't happen.
Bottom Line:
Bichel has his spot, and it is a question of Williams or Bracken in my opinion. I think Bracken's success against India may push him ahead, although one wonders who will open the bowling with William's pace gone.
It is a tough choice, and every bowler has every reason to be disappointed if he misses out.