That depends on how important you find 5Hz more low frequency extension and a few extra dB of SPL in the 80-200Hz range.
If you often run KH120 into limiting, but not the KH150, then you'd be fair to say the upgrade is 'big'.
However if you're never bottoming out the KH120, and especially if you anyway use it with a sub (as you should be using either of these), then the KH150 is not really better at all (for your use-case).
Actually, I suspect that for desktop ultra-nearfield listening (i.e. at modest SPLs) even the KH80 plus KH750 sub would be just as good, and perhaps even better - due to the fact that the smaller drivers integrate better at really close listening distances.
The main difference between these monitors is physical size, SPL and low frequency extension.
The engineering-minded approach is to 1) define your SPL requirements, and 2) select the smallest loudspeaker that satisfies them.