## Greatest rate of change? Partial derivatives

I have a function of two variables:

$f(x,y)=\frac{abx}{1-y+yx}$

Where a and b are constants, and I want to find out which variable, x or y, has the greatest impact on the rate of change of the function. I thought I could take the two partial derivatives, and prove that one is greater than the other for all values of x and y, but I'm not sure if this is the right approach, and if it is, I'm not sure how I'd prove it anyway.