Bolt wins easily. He split that 43.x clear back in February of this year, which was his "easy" year of training after an extended off season of promotional appearances and parties. The greatest sprinter of all time, in ANY sprint race, wins every time.
That's not to knock Rudisha, who is clearly a ridiculously amazing 800m runner. But the 800 is so much more controlled and less explosive compared to a 400, and Bolt's gonna have the huge advantage there. The backstretch 100 and the second corner 100 in a top world class 400 are covered in splits of around 10.3-ish, and I find it tough to see Rudisha moving that fast.