The Probability is 1/2. For 50% of the circle, A is > B. This is true for any circle that has origin at (0,0).
See picture below.
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Added Later:
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I stand corrected. I didn't notice the condition a and b are >0. As Manuu posted that this is only possible in 1st quadrant.
However with in the circle and with in the 1st quadrant, only half the time the x coordinate is greater than y coordinate, and vice versa.
So the probability is 1/8.