So I have had my 14 x 25 All Star for a little while and have used it invarious conditions from complete flat to 25knt down wind with a 3 metre swell and it absolutely flies. Then got my 12'6 x 25 about a month ago so have had a good chance to paddle that in most conditions also.
Taking a step back I am am 5'10 and 72kg, do 75% of my paddling on flat water through training 3 times a week and during the summer get out for atleast 1 down winder a week.
First thing you notice when you see these 2 boards side by side is how different they are. The 12'6 is a lot thicker through the whole board and of particular notice in the nose and tail. Although the Starboard catalogue says that the 12'6 is 223L it actually has 271L printed on it.
This makes the boards perform very differently. As we train in a quite tide effected area I actually find that the 12'6 feels faster going into the tide as it seems to sit above the water more than the 14, a bit like the Ace.
12'6 nose
14 nose
Having down winded both board a few times they again have similarities and differences. The 14 sits lower in the water and is more like an arrow piercing through the water and carries it's glide more while also requiring more foot work to move back on the tail. While the 12'6 with the extra volume in the nose seems to act a bit more like the Ace and doesn't need as much foot work.
12'6 rail profile
14 rail profile
They both are quite turnable using foot pressure without having to step back over the fin to surf it. After a couple of initial down winders I decided to swap the stock fin out as I felt the tail was quite lose and sometimes lifted and felt like it wanted to overtake the nose.
The tails are also quite different, obviously the 12'6 with the more volume is designed more with BOP in mind and having surfed it a few times in knee-waist high waves it handles them really well.
Unlike other companies the 12'6 is not just a mini me of the 14. Two very differnt boards designed for two different types of paddling and at 25 wide very stable.