As Charlie says, it depends what you want. I have been twice in Bansko, but never in Pamporovo. In my opinion Bansko is suitable for the beginning skier too, the ski way down to the gondola station is perfect.
Although it is becoming crowded in Bansko, it is still an authentic village and Papmporovo has been built as a resort.
I would recommend Bansko, but reading the comments and reports on this board, Pamporovo would not be a bad choice at all.