Inktense are brighter and permanent. If you want subtle colour you have to be very gentle. But if you allow them to dry between colourings you can layer colours without them muddying.
I have 36 Inktense and have toyed with the idea of getting some watercolours to fill out the gaps especially for the skin tones.
From what I can tell Inktense are unique so perhaps if you got them and still wanted to play with some regular water colour pencils, you could get some cheapies (not Faber Castel though - too hard) to play with. You couldn't do it the other way around.
There is no such thing as lack of sleep, only caffeine deficiency.