I considered putting this in the theory section, but it's based mostly on conjecture. Still, it's food for thought.
Of the three sannin, Tsunade is the only one whose student has yet to surpass her skills in any way. Naruto has surpassed Jiraiya's abilities by finally completing the rasengan, which no other user of the jutsu could do. Sasuke pretty much overpowered Orochimaru and stuffed his soul transfer jutsu. Of course, Jiraiya and Orochimaru had all sorts of great abilities their students don't have, but they were outclassed in certain areas.
Where does this leave Sakura? If Tsunade dies soon (which is a grand possibility), will she never get to see her pupil exceed her expectations? Tsunade is still clearly the physically stronger of the two. Sakura can only unleash her strength through punches and probably kicks as well. Tsunade, however, is strong enough throughout the entire body to have lifted Gamabunta's blade. I'm inclined, then, to believe that Sakura will exceed her sensei in the realm of medicine. Personally, I hope this will reveal itself through the restoration of Kakashi, which is the most urgent medical problem. Also, now that Tsunade is geared for battle, Sakura's medicine is likely to take the forefront.
In short, if Sakura's going to surpass her sensei, it's going to be soon.