Hi Bartc
To me there is a big difference. Watercolour pens aren't in the running. I find them really difficult to use. If you apply them directly onto paper, you are stuck with one dark value and a hard line to work with. I'm just not skilled enough for that. Pencils are much more forgiving.
And why Inktense pencils in particular? Four reasons:
1) Exactly what you mentioned - once an area has been wet and dried, it's permanent. With normal watercolour pencils I'm constantly churning up previous layers or areas of colour. (I tend to rework areas a lot). With Inktense I don't have to worry. I can put a light wash over a dark area and nothing moves.
2) Inktense does not fade as much when drying, which makes it easier to judge how dark to go.
3) This particular Inktense colour is quite transparent in light washes. It gives a clean feel. Dark watercolour pencils (and some other Inktense colours) are much more opaque and I don't like the effect.)
4) I absolutely love this particular colour. There is no similar colour in any of my other watercolour pencil sets (and I have several).
Hope that explains it. I use regular watercolour pencils (with watercolour paints) for landscapes, botanicals and insect drawings, but for this style of portrait, I really like the Inktense pencil.