Hi,
On a quality point of view, they really compare - the only reason I'm keeping my BigSky is the stereo input (using it for synths for instance).
Although I have to say that they do not sound exactly the same, for the same type of reverbs.
But if you are planning on using it only with the Kemper, I think it's a waste of money, considering that it's not cheap.
I have to say that I was stunned by the quality of the reverbs in the Kemper when they came out.