Thanks Hank for your reply.
We talked to our doctor about this and he explained us that although the numbing cream does have some negative effect on the treatment, the fact that the child is crying throughout the treatment (don't worry, it takes less then a minute :-)) overcomes this problem since the crying increases the blood flow in the area.
Moreover, the GA also has some negative effect since the child is lying totally relaxed and still.
Since we must choose for a child a painless way to do the treatment, it doesn't really matter which way we choose (referring only this aspect, of course)
It's not always true that the darker the PWS is the harder it will be to remove it. In our case the PWS is not dark at all but very stubborn...