Good point Mr S
I think that either Jodie or Rachel deserve to win. Not because they're older, but because they've been more consistent in the competition and they've demonstrated more of the full package needed for Nancy than the younger two. I also think there's a sense in which Rachel in particular has paid her dues and deserves a break ahead of the infants fresh from school.
However, I think that Sam or Jessie probably will win. Sam in particular seems to have the support of Andrew, the panel, and has come across as very likeable to the people at home.
While I like all of the final four, I think Sam might be the most disappointing winner in some ways. Physically, she's gorgeous for the role, but she would make the blandest Nancy. The big belt isn't particularly useful for the part and some of the richer alto voices (Keisha, Jodie, Jessie) would have added more. Her acting isn't brilliant, there are those that could have given more here too (Rachel).
The only unique thing Sam can offer the role is her youth. I'm sure there are lots of Sams at drama schools up and down the country, good voice, good mover. I'm sure lots of these girls are better actresses. Whereas with say Jessie or Jodie, you'd get a performer that you wouldn't have found in a drama school.
Kids coming from drama schools are very polished and Nancy should be rough round the edges. Jessie has a gorgeous voice and a lovely presence, but is very gauche and awkward, which would have been knocked out of her at drama school, but works well for this part. Jodie's huge personality suits the part too, but she would have been encouraged to tone that down.
I do really like Sam actually, but for this method of casting, I'm not sure she's my ideal winner.