Given the text, I'd be inclined to stick with your use of "difference". I'm assuming that the figure is not the outcome of some sort of statistical significance test but simply comparing the two correlation values.

"Ecart type" is "standard deviation" (the square root of the variance) but given the way "variance" is used as a statistical term, I think that it would be misleading to use it in the context of a simple difference between two figures.