Not to sure myself as it all depends... just to many variables.
If you want to hog a lot of material off to get to a particular shape the bigger the machine then the more you can hog!
Then you can invest in a very fast spindle machine and machine very very fast but that costs £s £s
Then again if the thing you want to machine is intricate then you sure aint cutting large amounts just using a smaller cutter, you then need a faster spindle.
The KX1 has faster spindle than the KX3, 7000 as to 5000, in my mind still not fast enough for small cutters.
I'm afraid your question has no real answer, you will have to decide what the ultimate requirement will be.
I have the X1 micro mill similar to the KX1 and so far I have not found any limitations in the mill, my skills on the other hand...
Have you also considered the implications of a much larger mill, a lot bigger and heavier, do you have the space for one.
I wish you well on your deliberations.
Peter