Since you asked, my thoughts on why I wouldn't play the 9 there:
-There's a much easier shot on the 2-ball. The 9-ball, even with the 4-ball backstop (and where would the 4-ball end up if you caromed off of it?) is a more missable shot. The cue ball is a little too far from the object ball to be comfortable when the easy 2-ball is begging to be played.
-The 2-ball really isn't a great key ball because it is too high above the side pocket to get to easily without a ball uptable leading to it. You could shoot it in the upper left corner, but it's a more precarious position play to that shot than to other types of key balls.
-With so many balls still on the table there's not a good enough reason to preserve the 2-ball for an unconventional break shot, especially since there's no key ball in position to help place the cue ball.
-It also looks like the 90 degree tangent line off of the 2-ball break ball misses the rack on the right side of the table, so cue ball placement and shot speed and spin would have to be very carefully played.
-The shot on the 9-ball is a little more than a half ball hit, which says to me that the speed on the cue ball necessary to get the 9-ball to the pocket will deflect the cue ball too strongly into the balls to know what would happen afterwards. It looks as if the cue ball would carom off of the 6-ball then 1-ball and then towards the 4-ball, though you might be able to put enough draw on the cue ball so that it strikes the 6-ball more fully and doesn't fly as much.
-Finally, the 9-ball is in a great position for a break ball for the next rack, so you may be able to preserve it.
My 2-cents.