why clock inverters over clock buffers?

why clock inverters  over clock buffers?

The factors which we consider are the equal rise time  and fall time, drive strength and the insertion delay of the cell. a perfect clock tree is that which have equal rise and fall times with 50% duty cycle for the clock. comparatively clock inverters will have less delay than buffers of same drive strength, also inverters. 

    If we replace a buffer with 2 inverter we can balance the clock rise and fall delay for longer length of the net.