Ok I have dug it out from my shelves.
It is a very thin paperback book containing artwork of various characters with a brief biography of them next to the picture. There are comments about some of the characters written by Terry. Things like what inspired him to create the characters.
I don't honestly think you'll gain any more knowledge about the characters from the Portfolio than you will from Turtle Recall.
Turtle Recall is actually an updated version of The Discworld Companion, so many of the entries in there are the same as the Companion.
And I just realised that I have a copy of Turtle Recall that I haven't even looked at yet.